REPORT  No.  53 


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PROPERTIES  AND  PREPARATION  OF 
CERAMIC  INSULATORS  FOR 
SPARK  PLUGS 


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REPORT  No.  53 


PROPERTIES  AND  PREPARATION  OF  CERAMIC 
INSULATORS  FOR  SPARK  PLUGS 

Part  I.— METHODS  OF  MEASURING  RESISTANCE  OF  INSULATORS 
AT  HIGH  TEMPERATURES 

By  F.  B.  SILSBEE  and  R.  K.  HONAMAN 

Part  II.— ELECTRICAL  RESISTANCE  OF  VARIOUS  INSULATING  MA- 
TERIALS AT  HIGH  TEMPERATURES 

By  R.  K.  HONAMAN  and  E.  L.  FONSECA 

Part  III.— PREPARATION  AND  COMPOSITION  OF  CERAMIC  BODIES 
FOR  SPARK-PLUG  INSULATORS 

By  A.  V.  BLEININGER 

Part  IV.— CEMENTS  FOR  SPARK-PLUG  ELECTRODES 

By  H.  F.  STALEY 


159827—20 1 


REPORT  No.  53. 

PART  I. 


METHODS  OF  MEASURING  RESISTANCE  OF  INSULATORS  AT  HIGH  TEMPERATURES.1 

By  F.  B.  Silsbee  and  R.  K.  Honaman. 

RfiSUME. 

This  report  describes  in  some  detail  the  preliminary  experiments  which  were  made  on  the 
conductivity  of  spark-plug  insulators  in  order  to  develop  a satisfactory  comparative  method 
for  testing  various  materials.  The  measurements  were  made  at  temperatures  between  200°  and 
900°  C,  and  with  both  alternating  and  direct  current  at  voltages  as  high  as  2,000  volts. 

The  results  obtained  confirmed  the  experiments  of  earlier  observers  at  lower  temperatures 
in  indicating  a very  rapid  decrease  of  resistance  with  increase  of  temperature  in  porcelain, 
mica,  fused  silica,  and  similar  materials.  This  decrease  is,  however,  a gradual  one  and  there 
is  no  definite  temperature  at  which  the  material  suddenly  changes  its  properties.  The  results 
of  conductivity  measurements  can  be  most  conveniently  expressed  by  stating  the  temperature 
( Te)  at  which  the  material  has  the  arbitrarily  selected  resistivity  of  one  megohm  per  centimeter 
cube.  Table  1 gives  the  values  of  this  constant  for  various  substances. 

The  measurement  with  direct  current  showed  the  presence  of  disturbing  polarization 
effects  which  make  the  apparent  resistance  of  the  specimen  vary  with  the  magnitude  and  time  of 
application  of  the  measuring  voltage.  This  effect  can  be  eliminated  by  the  use  of  alternating 
current  in  making  the  measurements  and  the  later  work  on  a wide  variety  of  substances,  the 
results  of  which  are  given  in  Part  II  of  this  report,  was  done  by  this  latter  method. 

There  is  a wide  field  for  further  investigation  of  this  subject,  as  the  mechanism  of  conduction 
in  this  class  of  materials  is  very  complex. 

INTRODUCTION. 

The  purpose  of  this  report  is  to  describe  some  measurements  carried  out  at  the  Bureau  of 
Standards  during  the  past  two  years,  on  the  resistance  of  various  insulating  materials  at  high 
temperatures.  This  work  was  undertaken  with  a view  to  studying  the  relative  merits  of  various 
insulators  for  use  in  spark  plugs,  and  in  particular  to  assist  the  ceramic  laboratory  of  the  bureau  in 
developing  improved  porcelain  bodies  for  this  purpose.  The  method  finally  adopted  as  a result 
of  this  work  for  the  comparative  testing  of  materials  is  described  briefly  in  Report  No.  51, 
Part  III,  the  results  of  a large  number  of  measurements  on  a wide  variety  of  materials  are  given 
in  Report  No.  53,  Part  II,  and  the  development  of  the  ceramic  side  of  the  investigation  is  given 
in  Report  No.  53,  Part  III.  The  present  report  will  be  confined  to  a description  of  the  various 
phenomena  observed  in  the  experiments  which  led  to  the  method  finally  adopted. 

The  electrical  and  thermal  conditions  under  which  a spark  plug  is  required  to  operate 
differ  considerably  with  the  type  of  gasoline  engine  used.  Measurements  with  embedded  thermo- 
couples have  shown  that  the  temperature  of  the  body  of  the  insulator  within  the  metal  shell 
seldom  exceeds  250°  C.  in  water-cooled  engines.  The  tip  of  the  inner  end,  however,  may  reach 
temperatures  as  high  as  900°  to  1,000°  C.  It  therefore,  appeared  desirable  to  study  the  resis- 
tivity of  the  specimens  in  the  range  of  temperature  between  200°  and  900°  C. 


1 This  Report  was  confidentially  circulated  during  the  war  as  Bureau  of  Standards  Aeronautic  Power  Plants  Report  No.  18. 


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The  electrical  stresses  applied  to  a spark-plug  insulator  by  the  average  magneto  or  battery 
coil  ignition  system  used  for  firing  gasoline  engines  are  quite  peculiar  and  difficult  to  duplicate  in 
any  method  of  measurement.  The  cycle  of  operation  (see  Report  No.  58,  Part  1)  following  the 
opening  of  the  primary  breaker  contacts  consists  of  a rapid  rise  of  the  potential  applied  to  the 
spark  plug  from  zero  to  a value  sufficient  to  break  down  the  spark  gap  in  the  engine  cylinder. 
The  break-down  voltage  is  of  the  order  of  6,000  volts  and  is  reached  in  a few  hundred  thou- 
sandths of  a second.  After  this  a comparatively  low  voltage  (800  volts)  maintains  the  electric 
arc  between  the  spark  points  and  lasts  for  a few  thousandths  of  a second.  Since  the  interval 
between  sparks  is  of  the  order  of  0.1  second,  it  will  be  seen  that  the  average  voltage  applied  over 
a complete  cycle  is  quite  low  and  has  been  found  to  be  approximately  150  volts.  These  peculiar 
electrical  conditions  should  be  kept  in  mind  when  considering  the  various  methods  of  measure- 
ment described  below. 

The  materials  studied  in  this  investigation  included  porcelains,  glass,  steatite,  mica,  and 
fused  silica,  as  these  constitute  the  only  class  of  substances  sufficiently  heat  resisting  for  use 
in  spark  plugs.  While  the  detailed  studies  of  polarization,  etc.,  described  in  this  report  were 
made  on  only  a few  of  the  porcelain  samples,  the  same  effects  seemed  to  be  present  to  greater 
or  less  degree  in  all  cases  and  the  process  of  conduction  is  probably  similar  in  all  of  them.  The 
work  of  earlier  investigators1  has  shown  the  complex  nature  of  the  phenomena,  but  as  yet  no 
complete  and  satisfactory  theory  has  been  worked  out  to  account  for  them. 

APPARATUS  AND  SPECIMENS. 

Most  of  the  work  reported  in  this  paper  was  done  on  cup-shaped  specimens  similar  to  the 
standard  test  piece  No.  1,  adopted  by  the  A.  S.  T.  M.,  except  that  the  side  walls  were  of  uniform 
thickness.  The  cross  section  of  this  specimen  is  shown  in  figure  1.  The  principal  advantages 
of  this  type  of  specimen  are: 

(1)  The  conduction  takes  place  through  the  bottom  of  the  cup,  which  is  of  definite 

and  easily  measured  dimensions. 

(2)  The  large  area  and  small  thickness  of  the  bottom  insure  a relatively  large  current 

even  with  material  of  high  resistivity. 

(3)  The  path  over  the  rim  of  the  cup  for  any  surface  leakage  is  relatively  long. 

(4)  A satisfactory  contact  can  be  made  between  the  specimen  and  the  electrodes  by 

immersing  the  bottom  of  the  cup  in  a conducting  fluid  (in  these  experiments, 
melted  solder)  and  by  inserting  some  of  this  fluid  inside  the  cup  to  form  the 
upper  electrode. 

These  cup  specimens  were  used  in  the  furnace  shown  in  figure  2.  The  heating  coil  inserted 
in  the  plug  below  the  specimen  was  found  necessary  to  compensate  for  the  flow  of  heat  through 
the  bottom  of  the  furnace.  By  proper  adjustment  of  the  relative  amounts  of  current  through 
the  main  winding  and  through  this  additional  coil,  the  temperatures  inside  and  outside  the  cup 
could  be  equalized.  These  temperatures  were  measured  by  two  copper-constantan  thermo- 
couples, one  of  which  was  inserted  in  a closed  porcelain  tube  which  dipped  below  the  surface 
of  the  solder  in  the  interior  of  the  cup,  while  the  other  was  embedded  in  the  steel  cup  containing 
the  solder  below  the  specimen.  Readings  of  the  resistance  were  taken  only  when  these  two 
thermocouples  showed  substantially  equal  temperatures. 

In  cases  where  cup  specimens  were  not  available,  measurements  were  made  on  assembled 
spark  plugs,  and  also  on  spark-plug  insulators,  and  on  short  pieces  of  tubing.  In  these  cases 
the  conduction  took  place  between  a central  electrode  and  either  the  shell  of  the  spark  plug 
or  a band  of  platinum  deposited  around  the  center  of  the  outside  of  the  insulator  or  tube.  The 
measurements  with  this  type  of  specimen  were  definite  in  indicating  the  resistance  of  the  speci- 
men, but  owing  to  the  uncertainty  as  to  the  area  of  contact  and  the  location  of  the  lines  of  cur- 
rent flow  it  is  difficult  from  such  data  to  compute  with  accuracy  the  true  resistivity  of  the 
material. 

1 Gray,  T.,  Phil.  Mag.  ser.  5,  vol.  10,  p.  226, 1880. 

Haworth,  H.  F.,  Proc.  Roy.  Soc.  Lond.  A 81,  p.  221,  1908. 

Somerville,  A.  A.,  Phys.  Rev.  31,  p.  261,  1910, 

Campbell,  Nat.  Phys.  Lab.  11,  p.  207,  1914, 


Kinnison,  C.  S.,  Proc.  Am.  Ceramic  Soc.  17,  p.  422,  1915. 
Poole,  H.  H.,  Phil.  Mag.  34,  p.  195,  1917. 

Brace,  P.  H.,  Trans.  Am.  Electrochem.  Soc.,  May  5,  1918, 


PROPERTIES  AND  PREPARATION  OF  CERAMIC  INSULATORS  FOR  SPARK  PLUGS. 


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Fig,  4, — Connections  for  high  voltage  D.  C.  measurements. 


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PROPERTIES  A^D  PREPARATION  OF  CERAMIC  INSULATORS  FOR  SPARK  PLUGS. 


For  specimens  of  this  shape  the  electric  furnace  shown  in  figure  3 was  used,  and  tempera- 
tures were  indicated  hy  a single  thermocouple  placed  in  an  iron  plug  which  supported  the 
specimen. 

For  reducing  the  results  of  either  type  of  specimen  from  the  observed  resistance  to  a basis 
of  the  resistivity  of  the  material  the  factors  connecting  these  two  quantities  were  computed 
from  the  dimensions  of  the  specimens.  The  resistivity  is  obtained  hy  multiplying  the 
observed  resistance  hy  the  factor  K.  where  for  the  cup  specimen 

(i) 

and  d is  the  diameter  of  the  bottom  of  the  cup  and  t the  thickness  of  the  cup  in  centimeters. 
For  the  tubular  specimen 

K 2ttZ 

2.30  log10.B2  (approx.  2) 

Bi 

where  l equals  the  length  of  the  external  conducting  hand  measured  parallel  to  the  length  of 
the  specimen,  and  R2  and  Rx  are,  respectively,  the  external  and  internal  radii  of  the  insulator. 

In  most  of  the  work  the  resistances  were  measured  hy  reading  a voltmeter  connected 
across  the  specimen  and  an  ammeter  in  series  with  it  and  taking  the  quotient  of  these  values  as 
the  resistance.  As  will  he  seen  from  the  following,  a wide  variety  of  sources  was  used  to  provide 
the  applied  voltage,  and  the  indicating  instruments  were  correspondingly  varied  in  character. 

VARIATION  OF  RESISTANCE  WITH  TEMPERATURE. 

The  first  experiments  were  carried  out  with  an  applied  direct-current  voltage  of  about 
2,000  volts,  which  was  obtained  hy  rectifying  with  a kenetron  a 3,000-cycle  voltage  supplied 
from  a step-up  transformer.  The  connections  used  to  obtain  this  rectification  and  to  reduce  the 
fluctuations  in  the  resulting  continuous  voltage  are  shown  in  figure  4.  This  rather  complicated 
system  was  chosen  in  an  attempt  to  duplicate  to  some  extent  the  voltages  existing  in  ignition 
systems,  and,  although  this  source  of  voltage  was  later  abandoned,  the  data  obtained  with  it 
brought  out  the  salient  facts  in  regard  to  this  type  of  conduction.  The  most  striking  of  these 
facts,  as  verified  by  other  measurements  made  later,  is  the  very  rapid  decrease  in  resistance  of 
the  specimen  with  increase  in  temperature.  This  variation  amounts  to  aproximately  2 per  cent 
per  degree  centigrade  at  all  temperatures.  If  the  results  are  expressed  by  plotting  resistance 
vs.  temperature,  or  conductance  vs.  temperature  (see  plot  5),  the  resulting  curves  are  so  steep 
as  to  render  it  impracticable  to  express  the  data  over  an  extended  temperature  range  by  a 
single  curve.  It  is  found,  however,  that  by  plotting  the  common  logarithm  of  the  resistance 
against  temperature,  as  is  done  in  curve  Xo.  Ill,  plot  5,  and  in  plot  6 a convenient  line  of 
slight  curvature  is  obtained.  It  will  be  seen  from  this  plot  that  if  this  curvature  is  neglected, 
the  results  can  he  represented  approximately  by  the  equation 

Log  10R  = a — bt  (3) 

This  method  of  expressing  the  results  is  very  convenient  in  reducing  the  data  to  a basis  of 
resistivity,  since  combining  the  relation 

f>=  KxR  (4) 

with  equation  (3)  one  obtains 

Log10p  = a + log10Z—  b t = c — b t (5) 

In  this  equation  b and  c are  constants  of  the  material  and  are  independent  of  the  shape 
and  size  of  the  specimen  used.  Unfortunately,  however,  the  values  obtained  for  one  of  these 
constants  depends  very  markedly  upon  the  other,  so  that  a slight  error  in  one  will  cause  a com- 
pensating change  in  the  other.  They  are,  therefore,  not  well  suited  for  comparing  the  relative 
merits  of  the  different  materials  and  for  this  latter  purpose  it  has  been  found  convenient  to 
compute  an  “effective  temperature”  ( Te ),  which  is  defined  as  the  temperature  at  which  the 
183136—20 2 


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material  lias  the  arbitrarily  selected  resistivity  of  1 megohm  per  centimeter  cube,  and  which 
may  be  computed  from  the  equation 


(6) 


This  value  of  Tc  ranges  from  350°  in  the  poorer  grades  of  porcelain  up  to  800°  for  fused 
silica,  and  is  a convenient  index  of  the  value  of  the  material  as  an  insulator  at  high  temperatures. 

An  inspection  of  these  resistance-temperature  curves  shows  a complete  absence  of  any 
critical  temperature  at  which  the  material  undergoes  an  abrupt  change  in  its  resistance.  This 
shows  the  error  of  the  commonly  accepted  idea  that  porcelain  breaks  down  and  becomes  con- 
ducting at  a definite  temperature.  This  belief  probably  originated  from  experiments  in  which 
the  temperature  of  a porcelain  sample  was  gradually  raised  while  being  continuously  subjected 
to  an  applied  voltage.  The  effect  of  the  current  flowing  through  the  sample  in  such  cases 

would  be  entirely  negligible  up  to  a certain  temperature  at  which  the  power  supplied  by 

the  measuring  current  became  comparable  with  the  rate  at  which  heat  could  be  dissipated  to 
the  surroundings.  Owing  to  the  very  rapid  rate  of  change  of  resistance  with  temperature,  a 
very  slight  further  increase  in  temperature  would  materially  decrease  the  resistance  and  con- 
sequently increase  the  loss.  Unless  the  specimen  was  in  a position  to  give  off  heat  freely 

to  its  surroundings  the  temperature  would  rise  rapidly,  causing  a further  decrease  in  resistance, 
thus  leading  to  an  unstable  state  which  would  rapidly  cause  the  fusion  of  the  material  and  the 
passage  of  an  arc.  The  rapidity  of  change  of  resistance  with  temperature  makes  this  point  of 
instability  quite  definite,  provided  all  the  conditions  of  the  experiment  are  maintained  con- 
stant, but  this  apparent  critical  temperature  will  depend  very  greatly  upon  the  contact  between 
the  specimen  and  the  furnace,  upon  the  applied  voltage,  and  the  other  conditions,  so  that  this 
is  in  no  sense  a specific  property  of  the  material. 

The  magnitude  of  this  heating  effect  is  exemplified  by  the  behavior  of  a porcelain  sample 
tested  when  hot — for  example,  at  500°  C.  At  this  temperature,  the  resistance  of  a centimeter 
cube  of  ordinary  porcelain  is  about  100,000  ohms,  and  if  a voltage,  of  only  500  volts  per  milli- 
meter (i.  e.,  only  about  one-twentieth  of  that  required  to  puncture  it  while  cold)  be  applied, 
the  current  flowing  will  be  50  milliamperes  and  the  power  dissipated  in  the  sample  will  be  250 
watts.  This  will  suffice  to  raise  the  temperature  of  the  sample  at  a rate  of  about  100°  C.  per 
second  and  will  cause  its  rapid  destruction. 

This  heating  of  the  specimen  by  the  measuring  current  was  observed  on  numerous  occasions 
when  making  tests  at  2,000  and  1,000  volts,  and  in  each  case  the  samples  on  removal  from  the 
furnace  were  found  to  contain  one  or  more  spots  where  the  porcelain  had  been  fused  into  a 
glass  by  the  intense  local  heating.  In  the  later  work  at  lower  voltage  this  effect  was  not  present, 
and  readings  were  taken  only  when  the  current  was  substantially  constant. 

\ POLARIZATION. 

The  early  measurements  with  high  voltage  direct  current  showed  a number  of  puzzling 
discrepancies,  such  as  a variation  of  the  apparent  resistance  with  the  voltage  used  in  making 
the  measurement  and  with  the  time  of  application  of  this  voltage.  Plot  7 shows  this  varia- 
tion of  resistance  with  voltage  as  observed  with  a porcelain  cup  specimen.  Such  discrepancies 
indicated  the  presence  of  an  additional  phenomenon  to  be  reckoned  with,  which  in  the  absence 
of  definite  knowledge  as  to  its  origin  was  called  “ polarization,”  and  will  be  so  referred  to 
throughout  this  report. 

The  fundamental  manifestation  of  this  so-called  polarization  is  that  if  a constant  direct- 
current  voltage  be  applied  to  a specimen,  the  resulting  current  will  decrease  at  first  rapidly 
and  then  more  gradually,  as  is  indicated  in  plot  8.  The  reduction  in  current  is  often  equivalent 
to  an  increase  in  resistance  by  a factor  of  10  or  20.  If  the  specimen  is  allowed  to  remain  at  a 
high  temperature  but  without  applied  voltage  for  some  time,  the  effect  gradually  disappears, 
but  a considerable  time  is  required  to  accomplish  this.  The  disappearance  is  more  ranid  at 


Resistance  of  Cup  in  Megohms. 


PROPERTIES  AND  PREPARATION  OP  CERAMIC  INSULATORS  FOR  SPARK  PLUGS. 


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PROPERTIES  AND  PREPARATION  OF  CERAMIC  INSULATORS  FOR  SPARK  PLUGS. 


11 


high  temperatures  than  at  low.  Plot  9 gives  a record  of  the  variation  of  the  apparent 
resistance  of  a glass  beaker,  as  measured  with  1,000  volts  direct  current,  after  various  applica- 
tions and  removals  of  the  measuring  potential.  The  course  of  the  experiment  is  indicated  by 
the  arrows,  and  the  duration  of  each  period  of  application  or  of  rest  is  indicated  on  the  curve. 
The  lowest  and  highest  curves  give  the  resistance  as  observed  with  very  short  application  of 
the  testing  voltage  just  prior  to  the  polarizing  test  and  on  the  following  day,  respectively. 
The  apparent  permanent  increase  of  resistance  observed  with  this  specimen  serves  to  explain 
some  mysterious  results  obtained  at  an  earlier  date,  in  which  one  specimen  had  shown  an 
increase  of  resistance  to  more  than  twenty  times  its  initial  value  after  several  successive  tests. 
The  fact  that  an  appreciable  time  is  required  to  obtain  a reading,  even  with  quick  acting  direct 
current  indicating  instruments,  and  that  during  this  time  the  specimen  is  being  polarized,  is 
probably  a complete  explanation  of  the  variation  of  apparent  resistance  with  applied  voltage, 
shown  in  plot  7.  If  the  applied  direct-current  voltage  is  suddenly  reversed  after  a specimen 
has  become  polarized  to  a considerable  extent,  the  initial  current  in  the  new  direction  is  found 
to  be  approximately  equal  in  magnitude  to  the  original  current  and  much  greater  than  the  value 
immediately  preceding  the  reversal.  (See  plot  10.)  This  implies  a counter  E.  M.  F.,  and  an 
attempt  was  made  to  observe  such  an  effect  by  connecting  an  electrostatic  voltmeter  across 
the  specimen.  No  residual  deflection  of  this  meter  was  observed  when  the  supply  current  was 
removed,  even  after  long-continued  polarization  of  the  specimen.  This  result  is  to  some  extent 
in  contradiction  to  facts  mentioned  by  other  observers.2 

A magneto  having  alternate  distributor  points  of  the  same  polarity  connected  together 
was  also  used  as  a source  of  voltage  and  the  polarizing  effects  found  to  be  in  every  way  similar 
to  those  obtained  with  a steady  direct-current  source  of  the  same  average  voltage  (150  volts). 

When  alternating  current  is  applied  to  a fresh  specimen,  there  is  no  polarizing  effect  and 
the  current  remains  constant  indefinitely,  except  when  the  current  is  so  large  as  to  produce 
heating  of  the  specimen.  When  alternating  current  is  applied  to  a specimen  which  has  been 
previously  polarized  by  direct  current,  the  polarization  disappears  at  a more  rapid  rate  than  if 
the  alternating  current  had  not  been  applied. 

An  attempt  to  throw  light  on  these  complex  phenomena  was  made  by  applying  alternating 
and  direct  current  simultaneously  to  a specimen.  This  was  accomplished  by  connecting  a 
transformer  in  series  with  a generator.  By  opening  the  primary  circuit  of  the  transformer, 
or  the  field  of  the  generator,  either  source  of  E.  M.  F.  could  be  eliminated  without  opening  the 
circuit  or  interfering  with  the  current  flow  from  the  other  source.  The  alternating-current 
voltage  was  measured  across  the  transformer  terminals  with  a moving  iron  voltmeter,  and  the 
direct-current  voltage  by  a d’Arsonval  type  direct-current  voltmeter  across  the  generator. 
The  alternating  current  through  the  specimen  was  passed  through  the  moving  coil  of  an  electro- 
dynamometer, the  fixed  coil  of  which  was  excited  by  an  alternating  current  of  constant  magni- 
tude and  in  the  same  phase  as  the  alternating  voltage  applied  to  the  specimen.  The  direct 
current  through  the  specimen  was  measured  by  a direct-current  milliammeter  connected  in 
series  with  the  specimen  and  the  dynamometer.  With  this  arrangement,  each  pair  of  instru- 
ments measured  only  its  particular  component  of  the  resultant  current  and  voltage  and  was 
not  affected  by  the  presence  of  the  other  component.  Plot  11  shows  the  variation  with 
time  during  the  course  of  the  experiments  of  the  resistances  as  computed  from  the  two  com- 
ponents of  the  current.  In  this  experiment  the  maximum  value  of  the  alternating-current 
voltage  was  greater  than  the  direct-current  voltage,  so  that  the  resultant  voltage  applied  to 
the  specimen  reversed  in  sign  during  each  alternation.  Other  experiments,  in  which  the 
maximum  alternating  voltage  was  less  than  the  direct-current  voltage,  and  the  resultant 
voltage  was  consequently  unidirectional,  showed  substantially  the  same  effects. 

Throughout  the  experiments  the  temperature  was  held  as  nearly  constant  as  possible, 
but  a gradual  drift  of  resistance  will  be  noticed  which  can  be  accounted  for  by  a slight  change 
of  temperature.  It  appears  from  these  results  that  the  resistance  of  the  specimen  is  sub- 
stantially the  same  for  both  the  alternating  and  direct  current  for  all  states  of  polarization. 


! Maxwell,  J.  C.,  Electricity  and  magnetism,  § 271,  vol.  1,  p.  393,  3d  ed. 


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ANNUAL  REPORT  NATIONAL  ADVISORY  COMMITTEE  E0R  AERONAUTICS. 


Or,  in  other  words,  the  polarization  produced  by  the  direct  current  ofTers  resistance  to  the 
passage  of  the  alternating  current  and  the  depolarization  produced  by  the  alternating  current 
reduces  the  resistance  offered  to  the  passage  of  the  direct  current. 

When  alternating  current  alone  was  applied  to  a fresh  specimen,  the  power  factor  of  the 
circuit  was  found  to  be  substantially  unity.  If,  however,  the  specimen  had  recently  been 
polarized  by  the  application  of  direct  current,  the  power  factor  was  somewhat  reduced;  values 
as  low  as  0.9  having  been  observed. 

The  data  described  above  are  quite  insufficient  for  the  development  of  any  complete  theory 
of  this  polarization,  but  it  would  appear  that  the  assumption  of  a counter  E.  M.  F.  is  ruled  out 
both  by  the  experiments  of  combined  alternating  and  direct  current  and  by  the  difficulty  of 
imagining  a mechanism  capable  of  producing  a counter  E.  M.  F.  of  the  order  of  several  thousand 
yolts,  which  would  be  required  to  produce  the  observed  decrease  of  current.  A possible  explana- 
tion may  be  developed  on  a basis  of  the  migration  away  from  one  electrode,  of  the  ions  cairymg 
the  current,  thus  leaving  a scarcity  of  carriers  for  the  further  passage  of  current  in  the  original 
direction,  but  providing  a plentiful  supply  for  currents  in  the  reversed  direction.  Another 
suggested  explanation  is  the  formation  of  resisting  films  which  may  cover  a considerable  part 
of  the  area  of  the  electrodes  but  which  are  readily  removed  by  electrolysis  on  reversal  of  the 
current.  Tests  made  using  platinized  surfaces  as  electrodes  instead  of  the  melted  solder 
showed  no  difference  of  behavior.  It  may  be  noticed  in  this  connection  that  when  the  samples 
were  removed  after  cooling,  the  solidified  solder  adhered  firmly  to  both  surfaces  of  the  cups 
which  had  been  treated  with  direct  current,  but  could  be  very  readily  peeled  off  from  specimens 
which  had  been  tested  on  alternating  current. 

DISCUSSION  OF  METHODS  FOR  MEASURING  RESISTANCES. 

As  a result  of  the  data  obtained  in  the  preliminary  experiments  just  described,  it  was 
decided  to  adopt  as  the  most  satisfactory  method  for  the  rapid  comparison  of  different  types 
of  insulating  materials  the  volt-ameter  method,  using  alternating  current.  Under  these  condi- 
tions the  observed  resistance  is  substantially  independent  of  the  frequency,  voltage,  and  time 
of  application,  and  the  convenient  values  of  60  cycles  and  500  volts  were  adopted  for  the  later 
work.  Plot  6 shows  typical  results  obtained  by  this  method  and  indicates  the  agreement 
attainable  on  successive  runs  even  at  different  voltages.  It  should  be  remembered,  however, 
that  the  results  thus  obtained  are  for  the  material  in  the  unpolarized  state,  and,  when  in  actual 
use  in  ignition  systems,  the  material  may  show  a much  higher  resistance  to  the  unidirectional 
impulse  from  the  magneto. 

Of  other  possible  methods  for  such  work,  a bridge  method  using  direct  current  would  be 
objectionable  because  of  the  variable  amount  of  polarization  which  would  occur.  Attempts 
were  made  to  use  alternating  current  as  a source,  but  the  measuiements  are  complicated  by  the 
effect  of  stray  capacities  shunting  the  high  resistances  which  are  necessary,  and  the  time 
required  to  obtain  a balance  on  the  bridge  is  a serious  drawback  because  of  the  rapid  change 
in  the  resistances  to  be  measured  with  even  slight  drifts  of  temperature. 

The  megger,  while  extremely  rapid  and  convenient,  is  open  to  the  disadvantages  of  polari- 
zation and  to  the  fact  that  the  voltage  supplied  varies  very  considerably  with  the  resistance  of 
the  specimen  under  test. 

The  use  of  a magneto  in  place  of  alternating  current  as  a source  has  the  great  advantage 
that  it  approximately  duplicates  the  conditions  of  operation  in  the  engine.  The  magneto, 
however,  is  very  variable  in  its  ouput,  both  from  instant  to  instant  and  as  a result  of  permanent 
changes  in  the  magnets,  contact  points,  etc.  Moreover,  there  is  an  abrupt  change  in  the 
operation  of  the  machine  when  the  resistance  of  the  specimen  becomes  so  low  as  to  cause  the 
spark  in  the  safety  gap  to  cease,  and  also  the  total  variation  of  the  current  delivered  with  various 
resistances  in  the  circuit  is  comparatively  slight  with  this  type  of  machine. 

A method  involving  the  measurement  of  the  rate  of  loss  of  charge  from  a condenser 
connected  in  parallel  with  the  specimen  has  been  used  by  Cunningham.  This  method  imitates 
the  conditions  of  operation  much  more  closely  than  does  the  alternating  current  method,  but 


PROPERTIES  AND  PREPARATION  OF  CERAMIC  INSULATORS  FOR  SPARK  PLUGS. 


13 


not  as  perfectly  as  the  use  of  a magneto  as  a source.  The  principal  objections  are  the  very 
delicate  string  electrometer  which  is  required  and  the  necessity  of  recording  the  results 
pho  togi  aphic  ally . 

^ & TYPICAL  RESULTS  AND  CONCLUSION. 


The  following  table  gives  the  results  obtained  by  the  use  of  the  alternating  current  method 
on  a number  of  types  of  samples,  the  significance  of  the  various  constants  being  the  same  as 
those  defined  previously. 

Table  1. 


Material. 

C 

b 

Tc 

p at  500°  C. 

11.8 

0.0065 

°C. 

890 

340. 106 

11.2 

.0066 

790 

80 

12.1 

.0085 

720 

70 

11.5 

.0085 

650 

40 

10.2 

.0085 

490 

.80 

These  figures  show  a wide  variation  in  the  resistance  of  the  different  materials  but  a rather 
surprising  similarity  in  the  constant  b,  which  is  a measure  of  the  temperature  coefficient  of  their 
resistance.  It  should  be  mentioned  in  this  connection  that  while  successive  measurements 
with  alternating  current  on  a single  specimen  give  results  repeating  to  a few  per  cent,  yet 
measurements  on  different  specimens  of  material  which  are  supposed  to  be  identical  show  wide 
variations  in  resistivity,  amounting  in  some  cases  to  a factor  of  10.  This  fact  tends  to  indicate 
that  the  conduction  is  due  to  a considerable  extent  to  the  presence  of  small  amounts  of 
impurities  which  may  vary  greatly  in  amount  without  appreciably  affecting  the  composition 
of  the  material  as  a whole. 

It  appears  from  the  above  data  that  the  alternating  current  method  developed  is  very 
practical  and  convenient  for  comparative  measurements  on  samples  of  this  character,  but 
there  is  a very  wide  field  of  investigation  concerning  the  phenomenon  of  polarization  and  much 
interesting  work  may  be  done  in  developing  theories  as  to  the  precise  mechanism  by  which 
conduction  is  carried  on  in  this  class  of  materials. 


\ 


v i h % i i 

\[)  iVI|M- 


. 


' 


/ 


r 


REPORT  No.  53. 

part  n. 


ELECTRICAL  RESISTANCE  OF  VARIOUS  INSULATING  MATERIALS  AT  HIGH 

TEMPERATURES.1 

By  R.  K.  Honeman  and  E.  L.  Fonseca. 


RESUME. 

In  very  hot  high-compression  engines  spark  plugs  may  fail  because  the  core  becomes  a con- 
ductor of  electricity.  The  ignition  current  then  flows  through  the  body  of  the  plug  instead  of 
causing  a spark  at  the  terminals.  The  data  given  in  this  report  show  the  characteristics  of 
various  porcelains  and  other  materials  in  this  respect.  These  data  include  values  for  a large 
number  of  plugs  now  on  the  market  and  also  various  experimental  porcelains  produced  in  the 
laboratory  and  covering  a wide  variety  of  compositions. 

To  secure  definite  and  repeatable  results  on  this  property,  it  was  found  necessary  to  make  the 
measurements  with  alternating  current,  using  a 500-volt,  60-cycle  supply.  In  this  way  the  dis- 
turbing effect  of  polarization,  etc.,  is  avoided.  The  conductivity  of  this  class  of  materials  in- 
creases very  rapidly  with  temperature  according  to  the  law  of  compound  interest  at  a rate  of  about 
2 per  cent  per  degree  centigrade. 

The  most  convenient  basis  of  comparison  for  different  materials  is  the  effective  tempera- 
ture” (Te)  to  which  they  must  be  heated  in  order  to  reduce  then’  resistivity  to  a definite  value.  This 
value  is  arbitrarily  taken  as  one  megohm  per  cm.  cube.  A spark  plug  of  normal  design  with 
material  of  this  resistivity  would  have  a resistance  of  about  200,000  ohms.  This  value  is  only 
slightly  above  the  limit  at  which  the  usual  ignition  system  can  be  counted  on  to  fire  a.  plug.  The 
effective  temperature  thus  defined  varies  from  870°  C.  in  case  of  fused  quartz  (the  best  material 
tested)  down  to  280°  C.  for  some  kinds  of  glass.  Porcelains  have  been  developed  at  the  Bureau 
of  Standards  ceramic  laboratory  which  have  an  effective  temperature  as  high  as  800°  C.  Certain 
bodies  recently  developed  for  use  in  aviation  engines  have  temperatures  of  about  650  C.,  while 
the  majority  of  spark-plug  porcelains  have  500°  C.  A material  having  Te  less  than  400  C.  should 
be  used  only  when  the  design  of  the  plug  is  such  that  the  insulator  is  extremely  well  cooled. 

INTRODUCTION. 

The  measurements  described  below  form  a part  of  the  investigation  of  sparkplugs  undertaken 
by  the  Bureau  of  Standards.  A number  of  cases  were  reported  of  spark-plug  failures  which  oc- 
curred under  conditions  suggesting  that  the  insulating  material  had  become  conducting  at  the 
high  temperatures  existing  in  aviation  engines,  and  an  extensive  study  of  this  phenomenon  in 
various  insulating  materials  was  undertaken.  Later  information  combined  with  data  accumu- 
lated in  the  laboratory  indicates  that  this  cause  of  spark-plug  failure  is  not  as  common  as  was  at 
first  thought,  but  may  occur  in  very  hot  engines.  The  suitability  of  a material  for  use  in  spark 
plugs  should  therefore  not  be  judged  solely  on  its  insulation  resistance  while  hot. 

The  ceramic  laboratory  of  the  bureau,  in  connection  with  this  work,  undertook  to  develop  an 
improved  type  of  porcelain  which  should  be  more  satisfactory  for  use  in  spark  plugs.  As  a basis 
for  this  a large  number  of  samples  of  porcelains  of  various  composition  were  made  up  so  as  to 
obtain  data  over  as  wide  a field  as  possible,  with  the  idea  of  correlating  the  physical  properties 
of  porcelains  with  their  compositions  and  heat  treatment  and  of  obtaining  data  of  value  for 
many  lines  of  work.  The  ceramic  side  of  this  undertaking  is  described  in  Part  III  of  this 
report.  

i This  R eport  was  confidentially  circulated  during  the  war  as  Bureau  of  Standards  Aeronautic  Power  Plants  Report  No.  19. 

183136—20 3 15 


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The  early  measurements  of  electrical  conductivity,  which  were  made  with  direct  current, 
showed  the  existence  of  very  complex  phenomena,  such  as  polarization,  etc.,  and  it  was  found 
necessary  to  develop  an  alternating-current  method  for  testing  the  specimens.  The  various 
phenomena  observed  during  the  development  of  this  method  are  discussed  in  detail  in  Part  1 
of  this  report,  and  in  what  follows  only  the  method  finally  adopted  and  the  results  obtained  by 
it  will  be  given. 

METHOD  OF  MEASUREMENT. 

The  specimen  of  insulating  material  to  be  tested  is  heated  in  an  electric  furnace  to  the  desired 
temperature.  Sixty-cycle  alternating  current  at  500  volts  is  impressed  upon  the  specimen  be- 
tween two  suitable  electrodes.  A voltmeter  connected  between  these  electrodes  and  an  ammeter 
in  series  with  the  specimen  are  read  simultaneously  and  the  quotient  of  effective  voltage  divided 
by  effective  current  is  taken  as  the  resistance. 

Since  the  current  through  the  specimen  is  rather  small,  a very  sensitive  milliammeter  is  re- 
quired and  in  most  of  the  work  it  is  found  most  convenient  to  use  for  this  purpose  a dynamometer 
wattmeter.  The  fixed  coil  of  this  wattmeter  is  excited  by  a known  current  from  the  same  source 
of  supply  as  the  test  current  through  the  specimen.  The  moving  coil  of  the  wattmeter  is  con- 
nected in  series  with  the  specimen.  Such  an  arrangement  provides  a higher  sensitivity  than  can 
be  attained  with  commercial  milliammeters,  and  is  also  convenient  in  the  great  range  of  currents 
which  can  be  measured  by  using  various  exciting  currents.  The  connections  used  in  this  test 
are  shown  in  figure  1. 

Measurements  made  by  this  method  on  the  same  sample  are  found  to  give  results  agreeing 
to  a few  per  cent  when  different  voltages,  frequencies,  and  times  of  application  of  the  measuring 
current  are  used.  This  method  has  been  adopted  by  the  Bureau  of  Aircraft  Production  and  is 
recommended  in  their  specification  No.  28,017. 

Two  typos  of  test  specimen  have  been  used  in  this  work.  For  the  accurate  measurement 
of  the  conductivity  of  the  material,  a cup  specimen,  similar  to  the  American  Society  for  Testing 
Materials,  standard  test  piece  No.  1,  is  very  convenient,  though  it  is  not  essential  that  the  side 
walls  of  the  cup  be  tapered.  Figure  2 gives  a cross  section  of  this  type  of  specimen. 

The  test  cup  is  filled  to  a depth  of  about  2 cm.  (0.8  inch)  with  melted  solder,  which  forms  one 
electrode,  and  is  in  turn  set  in  a slightly  larger  shallow  steel  cup  containing  melted  solder,  which 
forms  the  other  electrode.  This  arrangement  insures  good  contact  between  the  electrodes  and 
the  porcelain.  To  avoid  cracking  the  cup  it  is,  of  course,  necessary  to  insert  the  solder  in  the 
solid  form  and  melt  it  by  gradually  increasing  the  furnace  temperature.  Figure  3 shows  the 
arrangement  of  the  cup  specimen  in  the  furnace.  Two  thermocouples  protected  by  porcelain 
tubes  are  inserted  in  the  solder  in  the  test  specimen  and  in  a hole  in  the  steel  cup,  respectively. 
Measurements  of  resistivity  are  made  only  when  these  two  couples  indicate  substantially  the 
same  temperature. 

In  cases  wdiere  cup  specimens  are  not  available,  actual  spark  plu^  insulators  are  tested, 
using  the  central  electrode  as  one  terminal  and  the  shell  of  the  spark  plug  as  the  other.  In  cases 
where  the  insulator  is  removed  from  the  shell,  a band  about  2 cm.  (O.d  inch)  wide  at  the  center 
of  the  length  of  the  insulator  was  coated  with  platinum  by  painting  it  with  a solution  of  platinic 
chloride  (PtCl4)  in  oil  of  cloves  and  then  heating  the  deposit  to  reduce  the  chloride  to  metallic 
platinum.  This  platinum  belt  is  used  as  the  outer  terminal,  and  makes  good  contact  with  the 
insulator.  The  specimen  is  then  inserted,  as  showm  in  figure  4,  in  an  electric  furnace,  in  which 
the  temperature  is  indicated  by  a thermocouple  placed  in  contact  with  the  shell  of  the  plug. 

Measurements  obtained  on  these  plug  specimens  are  accurate  as  indicating  the  resistance 
of  the  specimen,  but  owing  to  the  irregular  shape  and  the  uncertainty  as  to  the  area  of  outer 
electrode  in  actual  contact  with  the  porcelain  they  are  not  suitable  for  accurately  measuring 
the  resistivity  of  the  material.  We  have,  however,  computed,  from  the  usual  formula  1 for 

1 For  concentric  cylinders  of  length  L and  diameter  d,  and  da  the  factor  connecting  resistance  and  resistivity  is 

K ..  2itL  

(L°g,o^)2.3 

For  the  cup  specimens  of  diameter  D and  bottom  thickness  t 


PROPERTIES  AND  PREPARATION  OF  CERAMIC  INSULATORS  FOR  SPARK  PLUGS. 


17 


Figure  4. 


18 


ANNUAL  REPORT  NATIONAL  ADVISORY  COMMITTEE  FOR  AERONAUTICS. 


19 


PROPERTIES  AND  PREPARATION  OF  CERAMIC  INSULATORS  FOR  SPARK  PLUGS. 

current  flow  between  concentric  cylinders,  the  factor  K,  by  which  the  resistance  must  be  multi- 
plied to  give  the  resistivity  of  the  material,  and  from  this  factor  have  computed  the  resistivities 
of  the  plugs,  which  are  given  in  Tables  IV  and  V.  In  this  computation  it  has  been  assumed 
that  contact  was  made  over  the  entire  surface  covered  by  the  shell  of  the  plug,  and  consequently 
the  values  of  resistivity  computed  on  this  basis  are  too  high  if  this  contact  be  imperfect.  It 
should  also  be  noted  that  the  path  for  surface  leakage  is  much  shorter  in  the  case  of  the  plug 
specimen,  and  also  that  if  the  glaze  is  of  poorer  material  than  the  body  conduction  through 
this  glaze  will  reduce  the  apparent  resistivity.  In  the  cup  specimen,  on  the  other  hand,  the 
area  of  contact  and  thickness  of  the  bottom  of  the  cup  are  definite  and  easily  measured,  and 
any  possible  leakage  path  is  very  long. 

COMPUTATION  OF  RESULTS. 

The  change  of  resistivity  with  temperature  for  the  class  of  materials  tested  is  so  rapid 
that  it  is  impracticable  to  plot  resistivity  vs.  temperature  directly,  as  a scale  which  is  suitable 
at  one  end  of  the  temperature  range  becomes  extremely  crowded  at  the  other.  It  has  been 
found,  however,  that  by  plotting  the  logarithm  of  the  resistivity  against  the  temperature  a 
smooth  curve  slightly  concave  upward  is  obtained.  The  curvature  of  this  plot  is  quite  small, 
and  the  data  can  be  represented  within  the  range  of  temperatures  used  with  sufficient  accuracy 
by  a straight  line  1 which  most  nearly  fits  the  observed  points.  Plot  5 shows  a typical  curve 
obtained  on  a cup  specimen  and  also  shows  the  agreement  to  be  expected  on  repeating  the 
observations  on  the  sav/ie  sample  even  at  different  voltages. 

The  points  on  the  straight  line  give  the  relation 

Log  10R  = a—bt  (1) 

where  R is  the  resistance  of  the  specimen  in  ohms  and  t the  temperature  in  degrees  Centigrade, 
while  a and  b are  constants  of  the  curve.  Introducing  the  factor  K mentioned  above,  we  obtain 

Log10p  = a + \og10K-b  t = c -bi  (2) 

where  p is  the  resistivity  in  ohms  per  centimeter  cube  and  c a new  constant  of  the  material 
which  is  obtained  from  a by  the  equation 

c = a + log10  K (3) 

c and  b depend  upon  the  material  only  and  not  upon  the  shape  or  size  of  the  particular  specimen, 
and  from  a knowledge  of  these  constants  p can  be  computed  for  any  temperature  by  equa- 
tion (2).  Unfortunately  neither  c nor  b is  very  convenient  as  a figure  of  merit  for  the  material, 
since  a slight  error  in  either  will  greatly  affect  the  value  of  the  other.  We  have,  therefore, 
found  it  advantageous  to  compute  for  each  material  p 500,  which  is  the  resistivity  of  the  material 
at  500°  C.  Another  still  more  convenient  figure  of  merit  is  the  temperature  at  which  the  material 
has  the  definite  resistivity  of  one  megohm  per  centimeter  cube. 

This  can  be  computed  by  the  equation 

rji  ^ 6 

le~  b 

This  value  is  given  in  the  following  tables  for  each  material,  and  is  the  most  satisfactory  criterion 
of  its  value  as  an  insulator  at  high  temperatures. 

A physical  interpretation  of  the  fact  that  the  results  when  plotted  logarithmically  give  a 
straight  line  can  be  obtained  by  considering  that  the  conductivity  increases  with  temperature, 
according  to  the  law  of  compound  interest,  at  a rate  of  about  2 per  cent  (2.3 b)  per  degree 
Centigrade.  The  cumulative  effect  of  the  compounding  is  such  that  an  increase  in  temperature 
of  100°  corresponds  in  the  average  material  to  an  increase  in  the  conductivity  of  600  per  cent. 

i A slight  error  is  theoretically  introduced  by  this  method  of  representing  the  results,  since  the  poorer  materials  are  tested  at  lower  tem- 
peratures, where  the  curve  is  steeper.  This  is  entirely  negligible  for  the  purposes  for  which  the  results  are  to  be  used. 


20  ANNUAL  REPORT  NATIONAL  ADVISORY  COMMITTEE  FOR  AERONAUTICS. 

TABLES. 

The  results  of  the  measurements  are  given  in  the  tables  below.  In  each  table  the  first 
column  contains  a description  of  the  material,  the  second  column  gives  a number  arbitrarily- 
assigned  to  the  specimen  for  laboratory  references,  and  the  remaining  columns  the  value  of 
the  constants  b,  c,  TC)  etc.,  defined  in  the  preceding  section. 

Table  1 gives  the  results  on  the  experimental  porcelains  made  up  in  the  ceramic  laboratory  of 
the  bureau.  The  composition,  heat  treatment,  etc.,  corresponding  to  each  of  these  specimens, 
is  given  in  Tables  6 and  7. 

Table  2 gives  similar  data  on  cup  specimens  which  have  been  submitted  by  various  porce- 
lain manufacturers  in  connection  with  their  development  work. 

Table  2 gives  results  on  various  other  materials  which  were  obtained  in  the  form  of  cru- 
cibles, tubing,  etc.,  by  the  laboratory. 

Tables  4 and  5 contain  the  results  on  American  and  foreign  spark  plugs,  respectively. 

Table  6 gives  the  compositions  and  firing  temperatures  fin  terms  of  Orton’s  pyrometric 
cones)  of  the  various  materials  listed  in  Table  1.  The  various  calcines  mentioned  in  this  table 
are  described  in  Table  7. 

The  numerical  values  in  the  tables  are  in  each  case  the  average  of  the  results  of  all  the 
samples  tested  of  the  same  material,  but  in  many  cases  only  one  sample  was  tested.  Unfor- 
tunately, the  variation  in  Te  observed  in  different  specimens  of  what  was  supposed  to  be  the 
same  material  sometimes  amounts  to  as  much  as  50°  C,  and  this  variation  should  be  borne  in 
mind  in  comparing  the  results  on  different  materials. 

It  should  also  be  noted  that  where  both  cup  and  plug  samples  of  nominally  the  same  mate- 
rial were  tested  the  latter  usually  showed  decidedly  lower  resistance.  Three  explanations  for 
this  may  be  suggested : 

First.  The  material  is  made  up  by  a somewhat  different  process,  the  cup  specimens  being 
molded  under  slight  pressure  while  the  plugs  are  extruded  through  a die. 

Second.  Surface  leakage  and  conduction  through  the  glaze  may  have  been  present  in  the 
plug  specimens  and  not  in  the  cups. 

Third.  The  cup  specimens  being  especially  prepared  for  test,  may  have  been  made  up  with 
greater  care  than  the  plugs. 

CONCLUSIONS. 

The  following  conclusions  seem  evident  from  the  data  given  in  the  tables. 

Quartz  is  by  far  the  best  of  the  materials  tested  as  far  as  resistance  at  high  temperature  is 
concerned,  although  several  of  the  laboratory  porcelain  bodies,  such  as  77  and  78,  approach  this 
fairly  closely.  The  mica  plugs  show  fairly  high  resistivity,  but  it  should  be  noted  also  that  this 
material  loses  its  water  of  crystallization  at  temperatures  approaching  1,000°  C.  and  becomes 
very  soft  and  friable. 

Several  of  the  porcelains  recently  developed  in  this  country  for  aviation  work  are  very 
notably  superior  to  the  majority  of  the  porcelains  tested. 

The  steatite  and  Rajah  porcelains  from  Germany  are  not  notably  high  in  resistivity,  and 
therefore  the  very  high  reputation  as  spark  plug  insulators  which  these  materials  had  before  the 
war  was  apparently  not  due  to  their  resistivity.  The  French  porcelains  are  on  the  whole  not  as 
good  as  the  more  recent  American  bodies.  This  may  account  for  the  preference  for  mica  plugs 
shown  in  the  former  country. 


PROPERTIES  AND  PREPARATION  OE  CERAMIC  INSULATORS  FOR  SPARK  PLUGS 
Table  1. — Bureau  of  Standards  cups. 


21 


Cup  No.— 

b. 

c. 

Resistivity 
at  500°  C. 

P 500 

megohms. 

Effective 
temp.  °C. 
Tc. 



0.  0135 

13.  58 

6.8 

560 

17  

.0089 

9.  50 

. 11 

390 

18  

.0070 

9.09 

.39 

440 

22  

.0089 

9. 33 

.08 

370 

.0096 

10. 30 

.32 

450 

24  

.0085 

9.  27 

.10 

380 

28  

. 0085 

9. 04 

.062 

358 

32i  

.0084 

9. 34 

.14 

400 

35  

.0108 

11.02 

.42 

460 

36  

.0109 

10.  34 

.078 

400 

39  

.0125 

10.84 

.039 

390 

40  

. 0080 

9.  28 

.19 

410 

47 1 . 

.0091 

9. 62 

. 12 

400 

48  

.0105 

10. 26 

.10 

410 

49  

.0076 

9.  07 

. 19 

400 

50  T 

.0113 

10.  43 

.060 

390 

.0119 

10.  77 

.066 

400 

63  . 

.0090 

10.  85 

2.2 

540  | 

70  . 

.0097 

10.  46 

.41 

460 

72 1 . 

.0098 

10.  89 

.40 

500 

73  

.0081 

9.  64 

.39 

450 

74 1 

.0099 

12.  37 

26 

640 

77  

. 0065 

11.  17 

83 

800 

78  

.0066 

11.21 

81 

790 

79  

.0080 

11.  11 

13 

640 

88  

. 0092 

11.38 

6 

590 

94  l 

.0119 

13.  32 

23 

620 

95  2 

.0089 

11.58 

13 

630 

107  

. 0083 

9.  96 

.65 

480 

109  

.0069 

10. 18 

5.4 

610 

116  

.0071 

11. 18 

43 

730 

119  

. 0075 

11.51 

58 

730 

152  3 

. 0073 

11.03 

24 

690 

iso  

.0074 

10.  77 

12 

640 

i Average  two  determinations.  2 Average  five  determinations.  3 Average  three  determinations. 


Table  2. — Cups  from  manufacturers. 


Maker. 

No. 

6. 

c. 

Resistivity 
at  500°  C. 

P 500 

megohms. 

Effective 
temp.  °C. 
7'e. 

Jefferv  Dewitt 

2 

0. 0071 

9.82 

1.9 

540 

Jeffery  Dewitt 

3 

.0088 

10.  83 

2.7 

550 

Jeffery  Dewitt 

5 

.0091 

11.98 

27 

660 

Champion  Ignition  Co 

11 

.0083 

11. 11 

9. 1 

620 

Champion  Ignition  Co 

17 

.0075 

10.  57 

6.6 

610 

tierold 

451A 

.0092 

9.  65 

. 11 

400 

Frenehtown 

577 

.0076 

10.  24 

2.8 

560 

Frenehtown 

315 

.0114 

10. 12 

.026 

360 

Frenehtown 

280 

.0107 

9.  74 

.025 

350  , 

Frenehtown 

775 

.0080 

11. 10 

12 

640 

Frenehtown 

775  X 

.0080 

11. 14 

14 

640 

Frenehtown 

775BG 

.0084 

10. 76 

3.6 

570 

Frenehtown 

2SBG 

.0087 

10. 10 

. 56 

470 

Frenehtown 

783 

.0090 

10.20 

.50 

470 

Frenehtown 

741 

.0077 

10.  70 

7. 1 

610 

Frenehtown 

56 

. 0064 

9.  80 

4.0 

590 

Brunt 

150 

.0069 

9.  78 

2. 1 

550 

Brunt 

154 

. 0097 

11.81 

9.  1 

600 

Jeffery  Dewitt  (supply  Champion  Toledo  plugs).  Frenehtown  (supply  Bethlehem  and  other  plugs).  Champion  Ignition  Co.  (supply  A.  C. 
Titan  plugs). 


Table  3. — Miscellaneous  specimens. 


Material. 

b. 

c. 

Resistivity 
at  500°  C. 

P 500 

megohms. 

Effective 
temp.  °C. 
Tc. 

0.  0081 

10.  43 

2.4 

550 

.0094 

9.56 

.072 

380 

.0092 

9. 14 

.035 

340 

.0214 

13.5 

. 00063 

350 

.0065 

11.7 

282.0 

880 

.0049 

10. 28 

68 

870 

.0034 

8.35 

4.5 

690 

ANNUAL  REPORT  NATIONAL  ADVISORY  COMMITTEE  FOR  AERONAUTICS. 


22 


Table  4. — American  plugs. 


Plug. 

No. 

b 

a 

c 

Logio  K. 

Resistivity 
at  500°  C. 

P 600 

megohms. 

Resistance 
at  500°  C. 

R 500 
megohms. 

Effective 
Temp. 
°C.  Te. 

PORCELAIN. 

Bethlehem 

611 

0. 0070 

8.97 

9.  69 

0.  72 

1.55 

0.  36 

530 

Bethlehem  Aviation 

1668 

.0061 

7.82 

8.  40 

.58 

.22 

.059 

390 

Berkshire 

12S 

.0095 

9. 05 

9. 47 

.42 

.052 

.020 

360 

(3141)  Champion 1 

231-041 

. 0078 

9.  32 

9.  77 

.45 

.74 

.26 

480 

. 00S2 

9. 63 

10. 00 

.37 

.79 

.34 

490 

Champion  (Toledo) 

Champion,  Toledo  (3450) 

1S12 

. 00S8 

10.  74 

10. 99 

.25 

3.99 

2.19 

570 

1823 

.0061 

8.  56 

8.81 

.25 

. 58 

. 32 

460 

B.  S.  Champion,  Toledo 

3496 

.0053 

8.  77 

8.  92 

.15 

1.9 

1.32 

550 

Do  

351S 

.0032 

7.16 

7.  31 

.15 

.61 

.36 

410 

. 0042 

7.56 

7. 92 

.36 

.66 

.29 

460 

Red. 

.0124 

11.54 

12. 05 

.51 

.71 

.22 

490 

Do 

Gray. 

.0102 

10. 47 

11.25 

.78 

1.4 

.23 

510 

Do 

Hercules. 

.0077 

10. 15 

10.  81 

.66 

9.1 

1.99 

620 

. 0065 

8. 10 

8.  37 

.27 

.13 

.07 

360 

Do  

C— 4 

.0086 

9.29 

9.56 

.27 

.18 

.098 

410 

Do.2 

C-5 

.0070 

8.87 

9. 14 

.27 

.44 

.23 

450 

Duffv 2 

.0069 

9.  IS 

9.  62 

.44 

1.5 

.54 

520 

Frenchtown 

Beth.  Avia. 

.0093 

10. 67 

10.74 

.064 

1.23 

1.05 

510 

Do 

56 

. 008S 

11.23 

11.48 

.25 

12.0 

6.8 

620 

Do 

S84 

.0066 

8.50 

9.  07 

,57 

.59 

.16 

470 

1818 

.0057 

8. 69 

9.  35 

.66 

3.36 

.69 

590 

793-022 

.0084 

9.51 

10. 10 

.66 

.91 

.20 

500 

. 0087 

9.  52 

10.  01 

.49 

.46 

. 15 

460 

45 

.0075 

9.22 

9. 79 

.57 

1.0 

.29 

500 

. 0078 

8.61 

8.  68 

.07 

.060 

. 05 

340 

Rajah  (Empire) 2 

45 

.0071 

8.30 

8.47 

.171 

.083 

.056 

350 

( Poors)  

.0080 

S.  86 

8. 93 

.07 

.085 

.072 

370 

. 056 

.0076 

7.08 

7. 74 

.66 

.008 

.0019 

230 

Titan  A.  C 2 

15. 458 

.0079 

8.  57 

9.  21 

.63 

.-18 

.04 

410 

Cico  A.  C 

.0052 

7.01 

7.63 

.61 

.117 

.026 

310 

Valve  olue 

. 028 

. 0056 

8.50 

8.  61 

. 11 

.65 

.50 

470 

W aldeh-W  orcester 

MICA. 

N7 

.0069 

8. 30 

8. 75 

.45 

.20 

.07 

400 

Splitdorf 

Splitdorf,  G.  J 

Bethlehem 

873 

.0078 

9. 08 

10.  2S 

1.20 

2.4 

.15 

550 

1819 

.0078 

9. 59 

10.  79 

1.20 

7.8 

.15 

614 

397 

.0143 

13. 73 

15.  00 

1.25 

71.0 

3.80 

630 

Berkshire 

110 

.0053 

9.25 

9.  93 

.68 

19.0 

3. 9S 

700 

GLASS. 

294 

.0100 

8.88 

9.  35 

.47 

.022 

.008 

335 

653 

.0158 

10.  87 

12.  IS 

1.31 

.02 

.001 

390 

Do 

655 

.0128 

9.70 

11.01 

1.31 

.04 

.002 

390 

LAY  A. 

. 006*4 

8.51 

9.20 

.69 

1.00 

.20 

500 

G.  E". 

X 16 

.0058 

9.25 

10.17 

.92 

18.0 

.240 

720 

STEATITE. 

Herz  Bougie 

332 

.0098 

9.63 

10. 16 

.53 

.18 

.054 

420 

Do.  .A 

1827 

.0098 

11.83 

12.  37 

.54 

29.0 

8.  5 

655 

1 Average  of  eight  determinations.  4 Average  of  four  determinations. 

2 Average  of  two  determinations.  4 Pyrex  and  glass. 


Table  5. — Foreign  plugs. 


Plug. 

B.  S. 
No. 

Material. 

b 

a 

c 

Logio  K. 

Resistivity 
at  500°  " 

P 500 

megohms. 

Resistance 
at  500°  C. 

R.  500 
megohms. 

Effective 
temp. 
°C.  Te. 

T?  p ’V 

0. 0096 

9.51 

10.  53 

1.02 

0.  54 

0. 051 

470 

439 

do 

.0086 

9. 10 

10. 07 

.97 

.61 

.063 

470 

421 

.0085 

9.  39 

10.26 

.86 

1.0 

.14 

500 

Composition 

. 0066 

8.46 

9.19 

.76 

.78 

.14 

480 

Fer’t 

Porcelain 

.0078 

8.  57 

9. 15 

.58 

.18 

.047 

400 

1 Average  three  determinations. 


23 


PROPERTIES  AND  PREPARATION  OE  CERAMIC  INSULATORS  FOR  SPARK  PLUGS. 


Table  6. — Sparh  plug  compositions. 


B.  S.  body  No. 

Georgia  kaolin. 

Florida  kaolin. 

North  Carolina  kaolin. 

j Delaware  kaolin. 

Maine  feldspar. 

P.ct. 

P.ct. 

P.ct. 

P.ct. 

P.ct. 

16 

11.25 

11.25 

11  25 

11.25 

16.00 

17 

11.25 

11.25 

11.25 

11.25 

18.00 

18 

11  25 

11.25 

11.25 

11.25 

20. 00 

22 

11.25 

11  25 

11.25 

11.25 

28.00 

23 

11.25 

11.25 

11.25 

11.25 

30.00 

24 

12. 50 

12.  50 

12.  .50 

12.  .50 

16.00 

28 

12.  50 

12. 50 

12.50 

12.  50 

24.00 

32 

13.  75 

13.  75 

13.75 

13.75 

14.  .50 

35 

13.75 

13.75 

13.75 

13.  75 

20.50 

36 

13.7.5 

13.  75 

13.  7.5 

13.75 

22.50 

39 

13.75 

13.75 

13.75 

13.  75 

28. 50 

40 

15. 00 

15.00 

15.00 

15.00 

16.00 

47 

15.00 

15.00 

15.00 

15. 00 

30.00 

48 

12.  50 

12.50 

12.  50 

12. 50 

.24.00 

49 

12.50 

12.  .50 

12.  50 

12.50 

22.  00 

50 

12.  75 

12.75 

12.75 

12.75 

25.00 

55 

12.50 

12. 50 

12.  .50 

12.  50 

28.50 

63 

12.50 

12.  50 

12. 50 

12.50 

13. 50 

70 

12.50 

12.  .50 

12  50 

12.50 

16.40 

72 

10.00 

9.00 

9.00 

9.00 

22.00 

73 

10.00 

9.00 

9.00 

9.00 

17.60 

77 

1.25 

1.25 

1.25 

1.25 

78 

2.50 

2.50 

2.50 

2.50 

79 

2.50 

2.50 

2.50 

2.50 

88 

12.  50 

12.50 

12  50 

12. 50 

8.00 

94 

11.25 

11.25 

11.25 

11.25 

17.60 

95 

10.00 

10.00 

10.00 

10.00 

107 

109 

116 

10.00 
8.00 
12.  50 

10.  00 
8.00 
12. 50 

13.  .50 

13. 50 
9.  00 

14.00 

12. 50 

12.  50 

119 

12.50 

12.50 

12.  .50 

12.  50 

152 

. 7. 50 

7.50 

7. 50 

7. 50 



180 

6.87 

. 6.87 

27.  75 

27.50 



P.ct. 

39.00 
37  00 
35  00 

27.00 

25. 00 

34.00 
26.  00 
29.  00 

23.00 

21.00 

15.00 

24.00 

10.00 
5.00 

12.  00 


30.00 


42.00 

21.00 


30.00 

30.00 


Whiting. 

Tennessee  ball  clay 
No.  5. 

Kentucky  ball  clay 
No.  4. 

Calcined  Florida  kao- 
lin. 

Calcino-kaoline,  equal 
parts  of  four. 

Calcine  No.  3. 

Calcine  No.  19. 

Calcine  No.  20. 

CO 

d 

£ 

0 

a 

'a 

0 ! 

Calcino  No.  14. 

I 

j 

j 

c 

N5  ! 

P.ct. 

P.ct. 

P.ct. 

P.ct. 

P.ct. 



P.ct. 

P.ct. 

P.ct. 

P.ct. 

P.ct. 

P.ct. 

1 

1.50 

1.50 

i 



1.00 

1.00 

1.50 

1.50 

1.50 

1.00 

20.00 

22.50 

20.00 



35.00 





4.10 

28.50 

5.00 
5.00 
7. 50 
5.00 
5.00 
| 7.  .50 

36.00 

36.00 

"l.  50 
j 5.00 
5.00 
7.50 

4.40 

SS.OO" 

85. 00 

80.00 
75.00 



12.00 

4.40 

AI2O3 

33.00 

MgCO. 

10.00 

18.00 

; 4.00 
10.00 

4.00 

10.00 

fair 

20.00 

35.00 

1 

20.00 

20.00 

20.00 

iigco 

4. 58 

5. 00 

1 5. 00 

40.00 

AI2O3 

11.52 

1 1 

1 1 

1 

16  down. 

15  down. 

14  down. 

14  ball  over. 

13  half  over. 

15  down. 

14  half  over. 

Do. 

12  down. 

Do. 

Do. 

13  down. 

10  down. 

14  down. 

Do. 

Do. 

14  half  over. 

16  down. 

14  down. 

13  down. 

Do. 

Do. 

Do. 

Do. 

Do. 

15  half  over 

16  down. 

20  down. 

12  down. 

16  down. 

18  down. 

Do. 

16  down. 

20  down. 


Table  7. — Sparh  plug  calcines. 


B.  of  S.  calcine,  No. 

MgCOa. 

Kaolin. 

Flint. 

Calcines, 

AI2O3. 

Boric  acid. 

Calcining 

tempera- 

ture. 

Per  cent. 
39.50 
14.40 
23.85 
18.20 

Per  cent. 
60. 50 
44. 30 
76. 15 
56.00 
70.20 

Per  cent. 

Per  cent. 

Per  ct  nt. 

Cone. 

10 

12 

12 

13 

20 

16 

8 A 

41.30 

14  

25.80 

27.80 

2.00 

34.30 

65. 70 

REPORT  No.  53. 

PART  m. 

PREPARATION  AND  COMPOSITION  OF  CERAMIC  BODIES  FOR  SPARK  PLUG 

INSULATORS.1 

By  A.  V.  Bleininger. 


resume. 

In  airplane  engines  the  porcelain  of  the  spark  pings  is  subjected  to  severe  conditions, 
involving  high  temperatures,  sudden  heating  and  cooling  effects,  and  mechanical  stresses 
To  be  perfectly  suited  for  the  purpose,  the  material  must  remain  a good  electrical  insulator  at 
the  maximum  temperature  reached,  should  not  be  subject  to  permanent  volume  changes, 
should  possess  constant  thermal  expansion,  and  must  be  strong  and  tough.  , 

Porcelains  possessing  such  qualities  have  been  developed  m the  Pittsburgh  laboratory  of 
the  Bureau  of  Standards.  The  compositions  of  the  best  types  are  as  follows: 


Porcelain  No.  152. 


Georgia  kaolin 

Florida  kaolin 

North  Carolina  kaolin.  . . . 

Delaware  kaolin 

Calcine  No.  19 

Calcine  No.  14 


Per  cent. 
10 
10 
10 
10 
40 
20 


The  compositions  of  the  calcines  are: 


Kaolin 

Alumina 

Magnesium  carbonate  (precipitated) 

Potters  flint 

Boric  acid 


Calcine  No.  19. 

70.  20 

27. 80 


Calcine  No.  14. 
56.  00 


2.  00 


Porcelain  No.  194- 


Beryl 

Georgia  kaolin 

Florida  kaolin 

North  Carolina  kaolin.  . . 

Delaware  kaolin 

Potters  flint 


18.  20 
25.  80 


Per  cent. 
..  35.0 
..  12.5 
..  12.5 
..  12.5 
..  12.5 
..  15.0 


PORCELAINS  FOR  SPARK  PLUGS. 

The  average  commercial  porcelain  does  not  fulfill  the  conditions  required  for  spark-plug 
service  nor  for  any  other  conditions  where  high-tension  currents  are  employed  and  the  tem- 
perature is  considerably  above  atmospheric  conditions.  From  the  standpoint  of  the  electrical 
res' stance  at  the  temperatures  reached  in  airplane  engines,  the  feldspathic  porcelains  begin  o 
break  down  at  a rate  rapidly  increasing  with  temperature,  due  to  electrolytic  effects  This  is 
indicated  also  by  the  polarization  which  is  observed  in  using  direct  current.  The  leakage  thus 
taking  place  may  become  a serious  factor.  

1 This  -Report  was  confidentially  circulated  during  the  war  as  Bureau  of  Standards  Aeronautic  Power  Plants  Report  No.  23 

Zo 


26  ANNUAL  REPORT  NATIONAL  ADVISORY  COMMITTEE  FOR  AERONAUTICS. 

This  average  porcelain  is  likewise  not  constant  in  volume;  and  these  volumetric  changes 
are  greater  than  can  be  explained  solely  by  thermal  expansion.  Aside  from  the  thermal 
expansion,  the  quartz  content  of  porcelain,  when  heated  or  cooled,  is  subject  to  certain  modi- 
fications in  crystalline  structure,  accompanied  by  definite  volume  changes.  Thus,  the  form 
of  quartz  permanent  at  atmospheric  temperature  is  alpha  quartz,  which  inverts  to  beta  quartz 
at  570°  C,  a transformation  which  is  reversed  by  cooling.  On  the  other  hand  if,  in  the  firing 
of  porcelain,  the  quartz  is  inverted  to  its  high  temperature  form,  cristobalite,  there  is  again 
the  transformation  of  the  alpha  to  the  beta  modification  to  be  considered,  which  takes  place 
at  230°  C.  In  either  case,  volume  changes  are  unavoidable  in  all  porcelains  containing  free 
quartz  in  large  amoimts,  due  to  the  inversion  noted.  In  a good  spark-plug  porcelain,  the 
quartz  should  be  eliminated  from  the  compound  and  replaced  by  a substance  not  subject  to 
these  inversions. 

Another  defect  of  the  ordinary  porcelain  is  that  the  coefficient  of  thermal  expansion  is 
by  no  means  constant  at  different  temperatures.  It  may  be  19  x 10-6  between  30 — 200°  C 
and  5 X 10-6  between  400 — 500°  C. 

With  respect  to  mechanical  strength,  also,  great  variations  are  possible.  A series  of  tests 
of  commercial  electrical  porcelains,  conducted  at  the  Pittsburgh  laboratory  of  the  Bureau  of 
Standards,  showed  differences  in  the  modulus  of  elasticity  varying  from  1,600,000  to  6,000,000. 

For  the  purpose  of  studying  porcelains  from  the  standpoint  of  their  electrical  conduc- 
tivity at  the  high  temperatures  obtaining  in  airplane  ignition  systems,  a large  number  of 
typical  compositions  were  made  up  in  the  form  of  the  test  cups  illustrated  in  figure  2 of  Part 
II  of  this  report.  These  mixtures  were  usually  prepared  in  10-kilogram  batches,  carefully 
weighed  out  and  ground  wet,  in  porcelain-lined  ball  mills  for  three  hours.  The  suspension  of 
clay  was  pumped  into  a filter  press  to  remove  the  water,  and  then  kneaded  in  a mixing  machine 
to  the  consistency  required  for  shaping  the  mass  on  the  potters  wheel.  This  was  done  by  the 
use  of  plaster  molds  in  which  the  cups  were  “jiggered”  in  the  usual  manner  in  which  pottery 
is  made.  The  specimens  were  then  dried,  placed  in  fire-clay  containers  (saggers)  and  burned 
in  a down-draft  kiln  fired  with  natural  gas  to  the  finishing  temperature.  The  latter  was  con- 
trolled both  by  means  of  thermocouples  with  the  necessary  galvanometers,  and  pyrometric 
cones,  made  by  Edward  Orton,  Columbus,  Ohio.  Each  cup  was  examined  for  nonabsorption 
by  the  application  of  ink.  The  testing  of  the  specimens  was  done  at  the  Washington  labora- 
tory as  described  in  Part  II  of  this  report.  The  characteristic  expression  for  the  resistivity 
of  the  porcelains  is  the  Te  value  which  represents  the  temperature  in  °C.  at  which  a cubic 
centimeter  of  the  material  still  shows  a resistance  of  one  megohm. 

REPLACEMENT  OF  FELDSPAR. 

Upon  comparing  the  Te  values  of  the  different  porcelains  it  was  noted  that  there  exists 
no  definite  relation  between  the  composition  and  the  electrical  conductivity.  On  the  other 
hand,  the  higher  the  maturing  temperatures  of  the  porcelains,  the  higher  was  the  Te  value. 
This  is  practically  equivalent  to  saying  that,  roughly  speaking,  the  electrical  resistance  at 
higher  temperatures  is  the  greater  the  lower  the  feldspar  content,  since  small  amounts  of  this 
flux  make  it  necessary  to  carry  the  porcelain  to  a higher  burning  temperature.  The  relation, 
however,  is  not  well  defined,  as  may  be  observed  from  the  results  compiled  in  the  following  table : 

Table  I. 


No.  of  body. 

Feldspar. 

Maturing 
temperature 
in  cones. 

Te  value. 

16 

Per  cent. 
16 

16 

560 

17 

18 

15 

390 

18 

20 

14 

440 

22 

2.8 

i 14 

370 

23 

30 

13 

450 

1 Half  over. 


27 


PROPERTIES  AND  PREPARATION  OF  CEkAMIC  INSULATORS  FOR  SPARK  PLUGS. 

It  is  quite  evident  that  the  micro-structure  of  the  porcelains  is  an  important  factor  in  this 
connection,  since  it  can  not  be  immaterial  how  much  kaolin  or  quartz  has  been  dissolved  by 
the  fused  feldspar  and  how  much  sillimanite  has  been  formed.  The  evidence,  however,  was 
considered  sufficient  to  warrant  the  replacement  of  the  feldspars  by  other  fluxes,  the  oxides  of 
magnesium  and  beryllium  being  used  for  this  purpose. 

Owing  to  the  evolution  of  carbon  dioxide  during  the  firing  process  from  magnesite,  and 
the  artificially  prepared  basic  magnesium  carbonate,  and  the  very  large  shrinkage  accom- 
panying their  use  as  a flux,  it  was  decided  to  introduce  the  magnesia  in  the  form  of  a calcine; 
that  is,  a silicate  mixture  previously  fired  to  a point  close  to  vitrification.  The  mixtures  em- 
ployed for  this  purpose  correspond  to  the  formula:  MgO  A1203  2Si02;  MgO  A1203  4Si02,  and 
MgO  A1203  6Si02.  Of  these,  the  first  and  the  second  were  most  used  in  this  work.  The  prepar- 
ation of  the  calcines  consisted  in  dry  ball  mill  grinding  and  firing  the  mixture  made  up  into 
balls  with  just  sufficient  water.  After  calcination,  the  material  was  crushed  and  ground  and 
introduced  into  the  bodies  in  this  form.  The  beryllium  oxide  was  brought  in  through  the  use 
of  the  mineral  beryl  (which  has  the  general  composition  3BeO  A1203  6Si02)  without  any  previous 
treatment. 

With  the  use  of  magnesia  as  the  principal  flux,  the  electrical  resistance  and  hence  the 
value  Te,  was  found  to  increase  quite  decidedly,  though  with  no  well-defined  regularity  as  referred 
to  percentage  content  of  magnesia.  It  was  seen  again  in  this  connection,  that  the  structure  of 
the  porcelain  plays  an  important  part  especially  as  it  is  known  that  MgO  accelerates  the  crys- 
tallization of  sillimanite  most  vigorously.  Not  only  the  number  but  also  the  size  of  the  silli- 
manite  crystals  is  of  significance  in  determining  the  texture  of  the  body,  whether  it  is  to  be  fine 
grained,  glassy,  or  coarsely  crystalline.  The  rate  of  cooling  the  porcelain  is  likewise  of  import- 
ance, since  a rapid  drop  in  temperature  invariably  causes  the  structure  to  be  closer  and  of  more 
vitreous  character  than  when  a longer  time  is  taken  in  cooling  down  the  kiln.  For  this  reason 
smaller  kilns  are  to  be  preferred  to  lai’ger  ones,  since  they  require  less  time  in  cooling  at  a 
given  rate.  The  effect  of  magnesia  added  in  the  form  of  synthetic  silicate,  is  strikingly  shown 
by  the  high  Te  values  of  bodies  Nos.  77  and  78.  See  Table  IV. 

It  is  a fact  that  the  magnesium  silicates  show  electrolytic  effects  which  are  much  less 
prominent  than  when  feldspar,  an  alkali- aluminum  silicate  (K20  A1203  6Si02)  is  used  as  a flux. 
It  likewise  appears  that  the  higher  the  firing  temperature  of  the  porcelain  the  greater  its  elec- 
trical resistance,  at  temperatures  up  to  700°  C.,  or  somewhat  above  this  point. 

With  reference  to  the  use  of  beryllium  oxide  it  was  found  that  this  flux  behaves  similarly 
to  the  magnesia  in  showing  high  electrical  resistance  and  Te  values.  This  is  indicated  by  the 
following  table: 

° Table  II. 


No. 

Beryl. 

Kaolin. 

Flint 

Maturing 
temperature 
of  porcelain 
in  cones. 

Te 

193 

Per  cent. 
25 

Per  cent. 
50 

Per  cent. 
25 

12 

624 

194 

35 

50 

15 

11 

784 

195 

45 

50 

5 

11 

798 

It  is  evident  from  these  results  that  beryllium  oxide,  used  in  the  form  of  the  mineral  beryl, 
is  a valuable  flux  from  the  standpoint  here  under  consideration.  It  requires,  however,  careful 
temperature  control  in  firing,  since  the  beryllium  porcelain  is  subject  to  sudden  deformation 
as  the  vitrification  temperature  is  exceeded.  The  experiments  have  proven  that  beryllium 
oxide  is  worthy  of  consideration  for  the  production  of  such  porcelain,  and  the  high  Te  value 
obtained,  798,  is  exceedingly  promising.  The  firing  temperature  of  these  bodies  is  quite  low, 
from  cones  11-12. 

Another  interesting  fact  developed  with  reference  to  the  thermal  expansion  of  the  beryl- 
lium porcelain,  which  was  found  to  be  lower  than  that  of  the  feldspathic  bodies  commonly 
employed.  The  average  coefficient  was  found  to  be  1.63  X 10~6  for  the  temperature  interval 
26—200°  C. ; 2.95  X IQ-6  for  200—400° ; 3.60  X IQ-6  for  400—570° ; and  2.33  X 10~6  for  26—400°. 


28 


ANNUAL  REPORT  NATIONAL  ADVISORY  COMMITTEE  EOR  AERONAUTICS. 


REPLACEMENT  OF  QUARTZ. 

From  a general  study  of  porcelains,  it  appears  desirable  to  eliminate  the  quartz,  as  has  been 
pointed  out  in  a previous  paragraph.  Some  of  the  materials  available  for  this  purpose  are 
calcined  kaolin,  synthetically  prepared  sillimanite  (A1203  Si02),  and  sintered  or  fused  alumina 
and  zirconium  oxide.  These  substances  have  been  introduced  in  a number  of  compositions. 
The  effect  of  adding  calcined  kaolin  in  general  was  beneficial;  and  even  with  a feldspar  content 
of  13.5  per  cent  (body  No.  63),  a fair  Te  value,  540,  was  obtained.  This  particular  composition 
contained  50  per  cent  raw  kaolin,  13.5  per  cent  feldspar,  1.5  per  cent  calcium  carbonate,  and 
35  per  cent  calcined  kaolin.  The  maturing  temperature  of  this  porcelain  was  that  correspond- 
ing to  cone  No.  16.  It  is  evident  that  by  raising  the  content  of  calcined  kaolin  still  more  at 
the  expense  of  the  feldspar  a higher  Te  value  would  be  obtained.  The  introduction  of  plastic 
ball  clay  to  replace  kaolin,  invariably  and  in  all  types  of  bodies,  reduces  the  electrical  resistance 
within  the  temperature  range  here  under  consideration. 

The  use  of  fused  alumina  in  any  extensive  work  was  prohibited  by  the  lack  of  material  low 
in  iron  content.  The  commercial  substance  (alundum)  is  unsuited  for  this  purpose.  The  total 
amount  of  white  fused  alumina  available  was  not  more  than  2 pounds.  To  bring  about  the 
necessary  impervious  and  dense  structure  the  use  of  17.6  per  cent  of  feldspar  was  required.  A 
mixture  consisting  of  45  per  cent  kaolin,  17.6  per  cent  feldspar,  4.4  per  cent  calcine  No.  13, 
and  33  per  cent  of  alundum,  resulted  in  a very  tough  porcelain  which  showed  a Te  value  of  620. 
Calcine  No.  13  was  compounded  according  to  the  formula  MgO  A1,03  2Si02  from  84  parts,  by 
weight,  of  magnesium  carbonate  and  258  of  plastic  kaolin  from  Florida  and  Georgia.  Here 
again  the  reduction  of  the  feldspar  content  and  its  replacement  by  calcine  No.  13,  or  beryl,  or 
a combination  of  these  two  would  be  certain  to  raise  the  Te  value  considerably  and  at  the  same 
time  would  result  in  a porcelain  of  excellent  mechanical  properties.  The  cost  of  the  fused 
white  alumina  would  be  quite  high,  but  would  be  justified  under  the  circumstances.  Further- 
more, a single  calcine  could  be  readily  produced  by  combining  the  raw  materials  of  calcine 
No.  13,  the  kaolin  and  magnesite,  with  uncalcined  alumina  from  any  convenient  source,  and 
firing  the  mixture  to  a point  of  constant  volume,  which  could  be  accomplished  at  temperatures 
not  exceeding  cone  No.  18.  At  the  same  time  this  procedure  would  simplify  the  process  of 
preparation  very  considerably. 

Sillimanite  (Al203Si02)  is  a normal  component  of  all  hard  fired  porcelain:  and  this  con- 
stituent, if  not  present  in  the  form  of  large  crystals,  imparts  to  the  material  constancy  in 
volume  upon  heating,  lowers  the  thermal  expansion,  increases  the  refractoriness,  and  the  resist- 
ance to  sudden  heating  and  cooling.  For  this  reason  it  was  thought  desirable  to  produce 
the  mineral  synthetically  and  to  introduce  it  in  the  porcelain  composition  in  place  of  the 
quartz.  This  was  done  by  combining  70.20  per  cent  of  plastic  kaolin,  27.80  of  anhydrous 
alumina,  and  2 per  cent  of  boric  acid.  After  grinding  this  mixture  in  the  dry  state  in  a ball  mill, 
it  was  made  up  with  water  to  form  a plastic  mass  which  was  molded  into  balls  calcined  to  a 
temperature  corresponding  to  that  of  cone  No.  20,  or  approximately  1,530°  C.  At  this  tem- 
perature the  mass  sintered  to  a dense  structure,  subject  to  but  little  additional  shrinkage 
and  containing  only  a small  amount  of  uncombined  alumina.  The  boric  acid  was  added  to 
assist  in  bringing  about  the  necessary  shrinkage  and  closing  up  of  the  pores,  and  it  is  quite 
probable  that  most  of  this  constituent  is  volatilized  at  the  final  temperature  of  the  calcination. 
This  calcine  (No.  19)  was  then  crushed  separately,  passed  over  a magnetic  separator,  and  then 
ground,  together  with  the  other  materials  of  the  body  with  the  addition  of  water,  in  the  ball  mill. 
In  preparing  this  type  of  body  the  feldspar  was  eliminated  on  the  basis  of  the  results  discussed 
above,  and  replaced  by  a calcine  corresponding  to  the  formula  MgO  Al203  4Si02  (No.  14). 
This  porcelain  (No.  152)  was  composed  of  30  per  cent  of  kaolin,  10  per  cent  of  ball  clay,  40  per 
cent  of  sillimanite  calcine,  and  20  per  cent  of  fluxing  calcine  No.  14,  and  fired  to  a temperature 
of  cone  No.  16  or  higher.  The  resultant  material  had  excellent  mechanical  qualities  and  an 
average  Te  value  of  690.  The  structure  of  this  body  should  be  fine  and  dense,  a condition 
which  requires  quite  rapid  cooling  of  the  kiln.  Slower  cooling  results  in  the  formation  of  coarser 
sillimanite  crystals  which  cause  a structure  much  less  resistant  to  sudden  heating  and  cooling. 


PROPERTIES  AND  PREPARATION  OF  CERAMIC  INSULATORS  FOR  SPARK  PLUGS.  29 

The  preparation  of  this  body  may  be  simplified  by  the  combination  of  the  two  calcines  into 
one,  thus,  by  employing  the  composition  of  the  flux  No.  13  (MgO  A1203  2Si02)  the  combined 
calcine  would  be  composed  of  75.10  per  cent  plastic  kaolin,  8.77  per  cent  magnesite,  and  16.13 
per  cent  anhydrous  alumina.  The  boric  acid  content  has  been  eliminated.  Body  No.  152 
would  then  consist  of  30  per  cent  of  kaolin,  10  per  cent  of  ball  clay,  and  60  per  cent  of  the  com- 
bined calcine.  Wherever  the  working  conditions  permit  the  ball  clay  should  be  eliminated, 
and  an  effort  made  to  employ  the  body  containing  40  per  cent  of  plastic  kaolin,  which  can 
usually  be  done  by  allowing  the  mixture  to  age  before  molding  it. 

The  tendency  of  the  body  to  crystallize  may  be  diminished  also  by  replacing  the  fluxing 
calcine  in  part  by  beryl,  so  that  these  two  components  are  present  in  the  ratio  of  1:1.  If,  then, 
the  combined  fluiing  and  sillimanite  calcine  is  employed,  the  resultant  composition  would  be 
kaolin,  60.94  per  cent;  anhydrous  alumina,  18.39  per  cent;  boric  acid,  1.32  per  cent;  magnesite, 
4.58  per  cent;  beryl,  14.78  per  cent.  The  body,  as  before,  would  consist  of  60  per  cent  of  this 
calcine  and  40  per  cent  of  kaolin.  The  fusion  point  of  this  mixture  is  so  low  that  it  would  be 
quite  possible  to  eliminate  the  boric  acid  entirely.  At  the  same  time  the  vitrification  point  of 
the  body  is  lower  and  closer  to  the  normal  kiln  temperatures. 

Attention  might  be  called  to  the  fact  that  the  use  of  a siliceous  porcelain  is  not  objectionable 
from  the  electrical  standpoint  but  only  from  consideration  of  the  mechanical  strength,  resist- 
ance to  sudden  temperature  changes,  etc.  This  is  shown  by  the  results  upon  high  silica  porce- 
lains, bodies  Nos.  116  and  119,  which  show  a very  high  Te  value,  namely  730.  This  type  of  por- 
celain contains  50  per  cent  of  clay,  30  per  cent  of  free  quartz  (flint),  and  20  per  cent  of  magnesia 
calcine.  In  No.  116,  the  calcine  corresponds  to  the  formula  MgO  A1203  2Si02,  and  in  No.  119 
to  MgO  A1203  4Si02.  It  is  fair  to  state  those  porcelains  would  give  excellent  results,  considered 
from  the  dielectric  standpoint,  when  used  under  conditions  not  so  extreme  with  reference  to 
temperature  and  shock  as  is  the  case  with  spark  plugs  used  in  airplane  engines. 

Zirconium  oxide  was  used  only  in  two  porcelains,  due  to  the  comparatively  limited  supply 
available.  The  material  at  hand  was  zircon  which  contained  52.74  per  cent  of  Zr02  and  43.46 
per  cent  of  silica.  As  received,  the  mineral  was  high  in  iron,  which,  however,  was  eliminated 
by  treatment  with  chlorine  at  a temperature  of  800°  C.  In  this  manner  all  but  a trace  of  the 
iron  was  removed.  Owing  to  the  fact  that  these  zircon  porcelains  were  produced  comparatively 
early  in  the  work,  feldspar  was  employed  as  the  flux.  The  result  was  that  the  electrical  resist- 
ance, and  hence  the  Te  value,  was  low,  being  500  and  450,  respectively.  With  the  use  of  the  mag- 
nesia or  beryllium  fluxes  much  better  products  could  undoubtedly  be  produced.  The  mechan- 
ical strength  of  these  porcelains  was  excellent  in  every  respect.  The  composition  of  these  bodies, 
Nos.  72  and  73,  is  given  in  Table  IV. 

The  effect  of  the  quartz  in  porcelain  upon  the  thermal  expansion  of  the  body  is  shown  by 
the  results  compiled  in  Table  III. 

Table  III. 


Clay. 

Quartz 

(flint). 

MgO  cal- 
cine. 

Formula  of  MgO  cal- 
cine. 

Silli- 

manite 

calcine. 

Coefficient  of  thermal  expansion  x 10®. 

No. 

30-200  °C. 

200-400  °C. 

400-550  °C. 

30-400  °C. 

114 

116 

117 

119 

120 
152 

Per  cent. 
50 
50 
50 
50 
50 
40 

Per  cent. 
20 
30 
20 
30 
20 

Per  cent. 
30 
20 
30 
20 

Per  cent. 

MgO  A1203  6Si02 

Per  cent. 

19.45 

9. 35 

5.52 

13.99 

MgO  AI2O3  2Si02 

7.  34 

6. 11 

4.  68 

6.  68 

8.  93 

4. 43 

4.  05 

6.  51 

MgO  AI2O3  4SiOo 

19.61 

11. 13 

8.08 

15.03 

30 

20 

MgO  AI2O3  4Si02 

10.  45 

5.43 

4.  45 

7.74 

MgO  AI2O3  4Si02 

40 

3. 36 

4. 19 

4.  78 

3.  81 

It  is  at  once  evident  from  a comparison  of  these  figures  that  the  thermal  expansion  of  these 
magnesia  porcelains  increases  with  the  silica  content  of  the  body,  and,  at  the  same  time,  is 
subject  to  decided  variation  within  the  temperature  limits  of  30°-550°  C.  On  the  other  hand 
the  porcelain  in  which  the  clay  content  is  lower,  and  all  the  quartz  has  been  replaced  by  silli- 
manite, shows  both  the  lowest  thermal  expansion  and  the  least  variation  in  the  value  of  the 
coefficient.  However,  the  composition  is  not  the  only  determining  factor,  and  the  importance 
of  the  microstructure  must  be  realized. 


30 


ANNUAL  REPORT  NATIONAL  ADVISORY  COMMITTEE  FOR  AERONAUTICS. 


CONCLUSIONS. 


The  work,  the  results  of  which  have  been  given,  has  been  successful  in  showing:  First,  the 
injurious  qualities  imparted  to  electrical  porcelains  by  the  use  of  feldspar  as  a flux;  and  second, 
in  bringing  out  the  desirability  of  replacing  the  quartz  by  minerals  or  synthetically  prepared 
materials  which  are  more  constant  in  volume  when  heated.  The  remedial  procedures  advocated 
are,  hence:  (1)  The  replacement  of  feldspar  by  other  fluxes,  such  as  silicates  of  the  type 
MgO  A1203  2Si02,  or  MgO  A1203  4Si02,  or  other  silicates  of  beryllium  and  the  alkaline  earths, 
either  natural  or  prepared  artificially.  (2)  The  elimination  of  quartz  and  the  substitution  of 
substances  not  subject  to  inversions  or  other  volume  changes.  These  may  be  highly  calcined 
kaolin,  alumina,  zircon,  or  sillimanite,  either  natural  or  produced  synthetically. 

The  compositions  of  the  most  typical  porcelains  produced  in  this  work  are  compiled  in 
Tables  IV,  V,  and  VI,  and  the  Te  value  is  given  for  each  body,  so  that  these  tabulations  may  be 
consulted  for  detailed  information. 

All  of  the  electrical  measurements,  as  well  as  the  determinations  of  the  thermal  expansion  of 
the  porcelains,  have  been  made  in  the  Washington  laboratories  of  the  Bureau  of  Standards. 


Table  IV. 


Number  of  body. 

16 

17 

18 

22 

23 

24 

28 

32 

35 

36 

39 

40 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Kaolin 

45.00 

45.00 

45.00 

45.00 

45.00 

50.00 

50.00 

55.00 

55.00 

55.00 

55.00 

60.  00 

16  00 

18.00 

20.00 

28.00 

30.00 

16.00 

24.00 

14.50 

20.50 

22.50 

28.50 

16  00 

Potters  flint 

39.00 

37.00 

35.00 

27.00 

25.00 

34.00 

26.00 

29.00 

23-00 

21.00 

15.00 

24.00 

1.50 

1.50 

1.50 

1.50 

BalLlnv  

Maturing  temperature  in  cones 

16 

15 

14 

14 

13 

15 

14 

14 

12 

12 

12 

13 

Te  value" 

560 

390 

440 

370 

450 

380 

358 

400 

460 

400 

390 

410 

Number  of  body. 

47 

48 

49 

50 

55 

63 

70 

72 

73 

74 

77 

78 

79 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

60.00 

50.00 

50.00 

51.00 

50.00 

50.00 

50.00 

37.00 

37.00 

5.00 

10.00 

10.00 

30.00 

24.00 

22.00 

25-00 

28.50 

13.50 

16.40 

22.00 

17.60 

10.00 

5.00 

12.00 

1.00 

1.00 

1.50 

1.50 

1.50 

1.00 

5.00 

5.00 

15.00 

io  66 

16.66 

15.00 

20.00 

15.00 

20.00 

35.00 

4.10 

4.40 

22.50 

28.50 

85.00 

85  00 

80.00 

75.00 

36.00 

36  00 

Maturing  temperature  in  cones 

10 

14 

14 

14 

14 

16 

14 

13 

13 

13 

13 

i3 

13 

Te  value 

400 

410 

400 

390 

400 

540 

460 

500 

450 

640 

800 

790 

640 

Number  of  body. 

88 

94 

95 

107 

109 

116 

152 

180 

193 

194 

195 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Kaolin 

50.00 

45.00 

40.00 

47.00 

25.00 

50.00 

30.00 

69.00 

40.00 

40.00 

40.00 

8.00 

17.60 

14.00 

30.00 

42.00 

21.00 

30.66 

5. 75 

25.00 

15.00 

5.00 

10.00 

4.58 

8.00 

20.00 

10.00 

9.15 

10.00 

10.00 

10.00 

35.00 

20.00 

12.00 

4.40 

20.00 

18.00 

20.00 

40.00 

33.00 

il.52 

TWvl  ...  .......  

25.00 

35.00 

45.00 

Maturing  temperature  in  cones 

15 

16 

20 

12 

16 

18 

16 

20 

12 

11 

11 

590 

620 

630 

480 

610 

730 

690 

PROPERTIES  AND  PREPARATION  OP  CERAMIC  INSULATORS  FOR  SPARK  PLUGS, 
Table  VI. — • Compositions  of  calcines. 


31 


Calcine 

No. 

MgCo3. 

Kaolins. 

Flint. 

Calcined 

A1203. 

Boric  acid. 

Calcination 
tempera- 
ture cone. 

Per  cent. 

Per  cent. 
60.50 
44.30 
76.15 
56.00 
70.20 

Per  cent. 

Per  cent. 

Per  cent. 

10 

8-A 

41.30 

12 

12 

14 

19 

20 

25.80 

13 

27.80 

2.00 

20 

34.30 

65.70 

16 

REPORT  No.  53. 

PART  IV. 


CEMENTS  FOR  SPARK-PLUG  ELECTRODES. 

By  H.  F.  Staley. 


r£sum£. 


Considerable  trouble  has  been  caused  in  airplane  engine  work  through  the  breaking  of  the 

central  electrode  of  spark  plugs.  . , 

An  investigation  of  this  problem  by  the  Bureau  of  Standards  shows  that  in  many  cases  the 

cement  used  to  hold  the  nickel  electrode  wire  in  the  porcelain  is  of  such  a nature  that  it  rapidly 
eats  away  the  wire  through  oxidization,  when  exposed  to  the  high  temperature  of  the  engine 
cylinder.  A cement  composed  of  silicate  of  soda  and  raw  kaolin  has  been  found  to  give  the  least 

trouble  in  this  respect.  . , . . , 

In  cases  where  the  cement  holds  the  wire  firmly  in  the  porcelain  the  latter  often  cracks  when 
subjected  to  heat  due  to  the  difference  in  the  coefficients  of  thermal  expansion  of  the  wire  and 
the  porcelain.  The  breaking  of  the  porcelain  does  not  seem  to  be  due  to  leaky  spark  plugs  as 
has  often  been  supposed  to  be  the  case. 

On  account  of  the  difficulties  attending  the  use  of  any  form  of  cement  between  the  porcelain 
and  central  electrode,  the  elimination  of  the  cement  and  the  use  of  a mechanical  seal  at  the  top 
of  the  porcelain  is  greatly  to  be  desired.  In  such  a plug  only  a porcelain  strong  enough  to  safely 
withstand  the  resulting  stresses  should  be  used.  The  porcelain  recently  developed  by  the 
Bureau  of  Standards  is  believed  to  meet  these  requirements. 

CEMENTS  FOR  SPARK-PLUG  ELECTRODES. 


Considerable  trouble  has  been  experienced  in  the  operation  of  airplane  engines  due  to 
breaking  of  the  central  electrode  wires  in  spark  plugs  at  the  point  where  they  enter  the  porcelain. 

Examination  of  numerous  broken  spark  plugs  showed  that  at  the  point  of  failure  the  central 
electrode  wire  was  oxidized  practically  through  its  cross  section,  this  oxidization  extending  up 
into  the  spark  plug  a variable  distance,  depending  upon  the  permeability  of  the  cement  with 
which  the  wire  was  surrounded.  In  some  cases  the  cement  had  melted  and  run  down  along  the 
wire.  It  was  apparent  from  the  nature  of  the  failures  that  in  order  to  thoroughly  study  the  prob- 
lem, the  composition  of  the  electrode  wire,  porcelain  insulator  and  the  cement  used  to  secure 

the  wire  to  the  insulator,  must  be  investigated. 

The  material  in  common  use  for  central  electrodes  in  the  spark  plugs  bought  by  the  Gov- 
ernment for  airplane  service  is  known  as  “ 97  per  cent  spark  point  nickel  wire.”  The  following 

are  typical  analyses:  . 

Analyses  of  wire. 


Laboratory  No 

Nickel 

Manganese 

Iron 

Copper 

i This  Report  was  confidentially  circulated  during  the  war  as  Bureau  of  Standards  Aeronautic  Power  Plants  Report  No.  35. 


47134 

47135 

97.  0 % 

96.  9% 

1.5 

1.6 

0.8 

0.9 

0.4 

0.  2 

33 


34 


ANNUAL  REPORT  NATIONAL  ADVISORY  COMMITTEE  FOR  AERONAUTICS. 


Interval: 

25  to  200°  C-. 
200  to  400°  C 
400  to  600°  C 
600  to  340°  C 
25  to  840°  C . 


Thermal  expansion  of  wire. 


Average  coefficient. 

14X10"8 

16 

16 

20 

17 


In  the  composition  of  porcelain  insulators  several  spark-plug  manufacturers  use  modifica-- 
tions  of  Bureau  of  Standards  body  No.  152,  the  thermal  expansion  of  which  is  as  follows: 


Interval: 

30  to  200°  C.. 
200  to  400°  C 
400  to  520°  C 
30  to  400°  C . 
30  to  510°  C . 


Thermal  expansion  of  porcelain. 


Average  coefficient. 
....  3.36X10-° 
....  4.19 
. . . . 4.  78 
....  3.81 
....  4.06 


In  the  case  of  the  cements  used  between  the  wire  and  porcelain,  it  developed  that  most  of 
these  consisted  of  a mixture  of  silicate  of  soda  with  finely  powdered  solids,  which  were  supposed 
to  be  chemically  inert.  Barium  sulphate  was  the  solid  used  by  the  manufacturer  whose  plugs 
gave  the  most  trouble,  while  another  maker,  whose  plugs  are  used  in  large  quantities  for  air- 
plane work,  employed  finely  ground  silica.  Silicate  of  soda  cements  are  advantageous  for  this 
work  because  they  are  cheap,  can  be  worked  cold,  and  are  gas  tight  at  low  temperatures. 

To  determine  the  effect  of  various  cements  on  electrode  wires,  mixtures  were  made  of 
silicate  of  soda  and  typical  powdered  materials.  Small  pellets  of  these  mixtures  were  worked 
around  commercial  nickel  electrode  wires  and  after  drying  at  120°  C.,  the  wires  and  adhering 
pellets  were  heated  in  an  oxidizing  atmosphere  to  1,000°  C.,  which  approximates  the  tempera- 
ture at  the  tip  of  the  porcelain  in  an  airplane  engine  in  operation.  After  cooling,  the  pellets 
were  broken  off  and  the  wire  beneath  them  examined.  The  results  are  given  in  Table  I. 

From  Table  I,  it  appeals  that  the  solids  used  do  not  act  simply  as  inert  material.  In  the 
second  case  in  this  table,  for  instance,  the  reaction  probably  is  as  follows: 

Na20  • Si02  + BaS04  = Na2S04  + BaO  • Si02. 

The  reactions  in  the  other  cases  are  too  problematical  to  be  worth  discussion  here.  It  is 
sufficient  to  say  that  raw  kaolin  was  found  to  be  the  best  solid  for  use  in  spark  plug  cements 
since  it  absorbed  the  sodium  silicate  as  fast  as  it  melted  and  produced  an  impervious  mass. 
The  oxidation  is  greatly  accelerated  by  contact  with  fused  alkaline  substances. 

Attempts  were  made  to  seal  the  wires  into  the  porcelain  after  heating  by  means  of  glasses 
ranging  in  coefficient  of  thermal  expansion  from  that  of  the  porcelain  to  that  of  the  wire. 
None  of  these  attempts  have  so  far  been  successful,  for  in  cooling,  the  glass  pulled  away  either 
from  the  porcelain  or  from  the  wire.  Special  wire,  with  a coefficient  of  thermal  expansion 
about  equal  to  that  of  porcelain,  has  been  ordered.  These  experiments  will  then  be  repeated. 

As  a part  of  this  investigation,  a visit  was  made  by  a representative  of  the  bureau  to  a 
spark  plug  factory  where  all  porcelains  were  subjected  to  a sudden  heating  test,  in  which  the 
tip  of  the  porcelain  was  placed  in  the  flame  of  a Meker  burner.  The  porcelains  were  uniformly 
passing  this  test.  When  it  was  suggested  that  porcelains  containing  electrodes  be  tested  in 
the  same  manner,  it  was  found  that  45  per  cent  of  the  porcelains  cracked.  As  the  result  of 
actual  engine  tests  of  complete  plugs  at  the  bureau,  it  has  been  found  in  general  that  in  the 
cracked  plugs  the  cement  is  holding  well,  the  failure  being  due  to  the  difference  in  coefficient  of 
thermal  expansion  between  the  porcelain  and  nickel  wire,  which  sets  up  so  high  a stress  between 
the  two  parts  of  the  plug  that  one  or  the  other  has  to  give  way.  This  conclusion  is  confirmed 
by  the  fact  that  in  the  above  factory  tests,  on  the  55  per  cent  of  plugs  that  did  not  fail,  either 
cement  was  absent  from  around  the  wire  near  the  tip  of  the  porcelain  or  else  it  softened  and 
gushed  out  of  the  hole. 


PROPERTIES  AND  PREPARATION  OP  CERAMIC  INSULATORS  FOR  SPARK  PLUGS. 


35 


The  subject  of  gas  leakage  of  spark  plugs  in  relation  to  failure  of  the  cential  electiode  has 
also  been  studied.  While  considerable  work  has  been  done  in  determining  the  gas  leakage  of 
new  plugs  in  view  of  the  above  results,  it  appeared  advisable  to  determine  leakage  on  plugs 
that  hacT been  used  in  an  airplane  engine.  In  Table  2 are  given  the  results  of  examination  of  a 
typical  lot  of  40  plugs.  The  following  information  will  be  of  assistance  in  reading  this  table. 
Of  the  12  broken  plugs,  6 leaked  badly  and  6 did  not  leak  at  all.  Of  those  that  leaked  badly, 
4 contained  loose  wires  and,  in  the  case  of  one  of  these,  the  wire  could  be  pulled  out  easily. 
It  is  evident  that  tests  for  gas  leakage  on  new  plugs  give  no  indication  of  their  gas  leakage  in  use 
'and  that  this  leakage  does  not  necessarily  produce  broken  porcelains. 

In  conclusion  it  may  be  stated  that  on  account  of  the  difficulties  incident  to  the  use  of  cement 
between  the  electrode  wires  and  porcelains  in  spark  plugs,  the  elimination  of  the  cement  entirely 
and  the  use  of  a mechanical  seal  at  the  top  of  the  porcelain  is  greatly  to  be  desired.  The 
mechanical  stress  incident  to  the  use  of  such  seals  should  be  localized  at  the  top  of  the  plug. 
The  type  of  porcelain  developed  by  the  bureau  has  sufficient  strength  to  withstand  the  necessary 
stresses.  At  least  two  manufacturers  of  spark  plugs  are  following  up  the  suggestions  of  the 
bureau  along  this  line  and  tests  of  these  plugs  are  to  be  carried  out  as  soon  as  samples  are  received. 

Table  I. — Effect  of  cements  on  electrode  wires. 


Cement 

No. 


Sodium 
silicate  40° 
BA,  c.  c. 


Water, 
c.  c. 


Solid. 


Grams. 


Kind. 


Powdered  silica  - 


do 

Barium  sulphate . 


Effect  of  heating  to  1,000°  C. 


Oxidation  of  wire. 


Very  bad. 


do 

Eaten  through. 


Description  of  cement. 


Hard,  strong,  slightly  porous.  Part  of  material 
had  run  down  the  wire. 

Do. 

Part  of  material  had  melted  and  run  down  wire, 
leaving  a hard  blue  mass  behind. 


Wire  dipped  in  40°  BA  sodium  s: 
Clean  wire 


None 

do. . . 

jvery  bad. 

| do — 

do 

do 

do 

do 

do 

.Slight 

Do. 

Hard,  strong,  very  slightly  porous. 
Hard,  strong,  not  porous. 

Soft,  weak,  porous. 

Do. 

Soft  powdery  mass. 

Do. 

Hard,  weak,  porous. 

Do. 

Soft  weak,  porous. 

Medium,  hard,  weak,  porous. 


Table  2. — Gas  leakage  around  electrode  wires  in  spark  plugs  that  had  been  used  for  1 f hours  in  a Liberty  engine. 


Porcelain 

broken. 

Porcelain 
not  broken. 

Leakage 

bad. 

Leakage 

moderate. 

No 

leakage. 

Wire  loose. 

Wire  easily 
pulled  out. 

12 

i 6 

4 

1 

6 

28 

17 

12 

3 

9 

2 

8 

3 

Total  12 

28 

23 

8 

9 

19 

12 

1 Part  of  these  may  have  been  broken  while  the  plugs  were  being  removed  from  the  engines. 


o 


OF  lUAffiv  LLgA.i  ' 

SEP  5 1923 


• ' 

, 

■ 


■ 


■ 


' 


- 


, 


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■ 11 


